Component used: Navigation
Version used: 2.0.0
Devices/Android versions reproduced on: n/a

Android Studio 3.3.2
Build #AI-182.5107.16.33.5314842, built on February 15, 2019
JRE: 1.8.0_152-release-1248-b01 amd64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Linux 4.15.0-46-generic
com.android.tools.build:gradle:3.3.2
gradle-4.10.3-all

I have a destination with a nullable String argument:

<fragment
android:id="@+id/editFragment"
android:name="com.commonsware.todo.ui.edit.EditFragment"
android:label="@string/app_name" >
<argument
android:name="modelId"
app:argType="string"
app:nullable="true" />
</fragment>

I have an action that attempts to declare null as the default value:

<action
android:id="@+id/createModel"
app:destination="@id/editFragment" >
<argument
android:name="modelId"
android:defaultValue="@null" />
</action>

The generated SafeArgs code treats @null as a String:

private data class CreateModel(val modelId: String = "@null") : NavDirections {
override fun getActionId(): Int = R.id.createModel

override fun getArguments(): Bundle {
val result = Bundle()
result.putString("modelId", this.modelId)
return result
}
}

fun createModel(modelId: String = "@null"): NavDirections = CreateModel(modelId)

IOW, I am seeing similar symptoms to https://issuetracker.google.com/issues/111425916, except that I'm consistently using 2.0.0 (plugin and runtime libraries).

If I remove the default value, the SafeArgs-generated code goes back to supporting null, just without a default null:

private data class CreateModel(val modelId: String?) : NavDirections {
override fun getActionId(): Int = R.id.createModel

override fun getArguments(): Bundle {
val result = Bundle()
result.putString("modelId", this.modelId)
return result
}
}

fun createModel(modelId: String?): NavDirections = CreateModel(modelId)

Let me know if you need additional information -- thanks!