This is my code:

      override fun mergeItem(key: String, value: String): StorageResult<Nothing> {
        if (!ensureSetup()) {
            return StorageResult.Failure("Setup failed")
        }

        val changeManifest = atomic(false)

        val valueOld = getValueForKey(key)

        var newValue = value
        if (valueOld != null) {
            val mergedValue =
                Json.decodeFromString<MutableMap<String, Any>>(value)
            val entryValue =
                Json.decodeFromString<MutableMap<String, Any>>(valueOld.toString())
            if (mergeRecursive(mergedValue, entryValue)) {
                newValue = Json.encodeToString(mergedValue)
            }
        }
        writeEntry(key, newValue)
        if (changeManifest.value) {
            writeManifest()
        }
        return StorageResult.Success()
    }


 private fun mergeRecursive(
        destination: MutableMap<String, Any>,
        source: Map<String, Any>
    ): Boolean {
        var modified = false
        for ((key, value) in source) {
            val sourceValue = value
            val destinationValue = destination[key]
            if (sourceValue is Map<*, *>) {
                if (destinationValue is Map<*, *>) {
                    if (mergeRecursive(
                            destinationValue as MutableMap<String, Any>,
                            sourceValue as Map<String, Any>
                        )
                    ) {
                        destination[key] = destinationValue
                        modified = true
                    }
                } else {
                    destination[key] = sourceValue
                    modified = true
                }
            } else if (source != destinationValue) {
                destination[key] = sourceValue
                modified = true
            }
        }
        return modified
    }

Test case

    @Test
    fun testMergeAsyncStorage() {
        val map = HashMap<String, String>()
        map["key1"] = "value1";
        map["key2"] = "value2";
        map["key3"] = "value3";
        val jsonAbc = Json.encodeToString(map)
        AsyncStorage().setItem("jsonAbc", jsonAbc)
        val map1 = HashMap<String, String>()
        map1["key1"] = "value12";
        map1["key2"] = "value22";
        map1["key3"] = "value32";
        val jsonAbc1 = Json.encodeToString(map1)
        AsyncStorage().mergeItem("jsonAbc", jsonAbc1)
        val res =  AsyncStorage().getItem("jsonAbc")
        val map1Clone = res?.let { Json.decodeFromString<HashMap<String, String>>(it) }

        assert(map1["key1"] == map1Clone?.get("key1"))
    }

And is show this error:

kotlinx.serialization.SerializationException: Serializer for class 'Any' is not found.
Mark the class as @Serializable or provide the serializer explicitly.
On Kotlin/Native explicitly declared serializer should be used for interfaces and enums without @Serializable annotation
kotlinx.serialization.SerializationException: Serializer for class 'Any' is not found.
Mark the class as @Serializable or provide the serializer explicitly.
On Kotlin/Native explicitly declared serializer should be used for interfaces and enums without @Serializable annotation
    at kotlin.Throwable#<init>(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/Throwable.kt:24)
    at kotlin.Exception#<init>(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/Exceptions.kt:23)
    at kotlin.RuntimeException#<init>(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/Exceptions.kt:34)
    at kotlin.IllegalArgumentException#<init>(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/Exceptions.kt:59)
    at kotlinx.serialization.SerializationException#<init>(/opt/buildAgent/work/b2fef8360e1bcf3d/core/commonMain/src/kotlinx/serialization/SerializationException.kt:48)
    at kotlinx.serialization.internal#platformSpecificSerializerNotRegistered__at__kotlin.reflect.KClass<*>(/opt/buildAgent/work/b2fef8360e1bcf3d/core/nativeMain/src/kotlinx/serialization/internal/Platform.kt:22)
    at kotlinx.serialization#serializer__at__kotlinx.serialization.modules.SerializersModule(/opt/buildAgent/work/b2fef8360e1bcf3d/core/commonMain/src/kotlinx/serialization/Serializers.kt:60)
    at kotlinx.serialization.builtinSerializer#internal(/opt/buildAgent/work/b2fef8360e1bcf3d/core/commonMain/src/kotlinx/serialization/Serializers.kt:96)
    at kotlinx.serialization.serializerByKTypeImpl#internal(/opt/buildAgent/work/b2fef8360e1bcf3d/core/commonMain/src/kotlinx/serialization/Serializers.kt:84)
    at kotlinx.serialization#serializer__at__kotlinx.serialization.modules.SerializersModule(/opt/buildAgent/work/b2fef8360e1bcf3d/core/commonMain/src/kotlinx/serialization/Serializers.kt:59)
    at vn.momo.core.modules.storage.async.AsyncStorageModule#multiMerge(/Users/anhdevit/Desktop/momo-app-vix/shared/src/iosMain/kotlin/vn/momo/core/modules/storage/async/AsyncStorageModule.kt:385)
    at vn.momo.core.modules.storage.async.AsyncStorage#mergeItem(/Users/anhdevit/Desktop/momo-app-vix/shared/src/iosMain/kotlin/vn/momo/core/modules/storage/async/AsyncStorage.kt:23)
    at vn.momo.core.storage.async.iOSAsyncStorageTest#testMergeAsyncStorage(/Users/anhdevit/Desktop/momo-app-vix/shared/src/iosTest/kotlin/vn/momo/core/storage/async/iOSAsyncStorageTest.kt:115)
    at vn.momo.core.storage.async.$iOSAsyncStorageTest$test$0.$testMergeAsyncStorage$FUNCTION_REFERENCE$23.invoke#internal(/Users/anhdevit/Desktop/momo-app-vix/shared/src/iosTest/kotlin/vn/momo/core/storage/async/iOSAsyncStorageTest.kt:120)
    at vn.momo.core.storage.async.$iOSAsyncStorageTest$test$0.$testMergeAsyncStorage$FUNCTION_REFERENCE$23.$<bridge-UNNN>invoke(/Users/anhdevit/Desktop/momo-app-vix/shared/src/iosTest/kotlin/vn/momo/core/storage/async/iOSAsyncStorageTest.kt:120)
    at kotlin.native.internal.test.BaseClassSuite.TestCase#run(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestSuite.kt:85)
    at kotlin.native.internal.test.TestRunner.run#internal(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestRunner.kt:245)
    at kotlin.native.internal.test.TestRunner.runIteration#internal(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestRunner.kt:271)
    at kotlin.native.internal.test.TestRunner#run(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/TestRunner.kt:286)
    at kotlin.native.internal.test#testLauncherEntryPoint(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/Launcher.kt:30)
    at kotlin.native.internal.test#main(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/Launcher.kt:34)
    at <global>.Konan_start(/Users/teamcity1/teamcity_work/6326934d18cfe24e/kotlin/kotlin-native/runtime/src/main/kotlin/kotlin/native/internal/test/Launcher.kt:33)
    at <global>.Init_and_run_start(Unknown Source)
    at <global>.start_sim(Unknown Source)
    at <global>.0x0(Unknown Source)

Update: StorageResult

sealed class StorageResult<out T> {
    data class Success<T>(val data: T? = null) : StorageResult<T>()
    data class Failure(val errorMessage: String?, val throwable: Throwable? = null) : StorageResult<Nothing>()
}

import

import co.touchlab.stately.freeze
import co.touchlab.stately.isolate.IsolateState
import kotlinx.atomicfu.atomic
import platform.Foundation.*
import kotlinx.cinterop.*
import kotlinx.serialization.*
import kotlinx.serialization.json.Json
import platform.darwin.NSUInteger

    private fun getValueForKey(key: String): String? {
        var value = asyncStorageHolder.access {
            it.manifest[key]
        }
        if (value == null) {
            value = getCache().objectForKey(key) as String?
            if (value == null) {
                val filePath = filePathForKey(key)
                val resultReadFile = readFile(filePath)

                value = when (resultReadFile) {
                    is StorageResult.Success -> {
                        resultReadFile.data
                    }
                    is StorageResult.Failure -> {
                        null
                    }
                }
                getCache().setObject(value, key, value?.length as NSUInteger)
            }
        }
        return value
    }