Mysterious Kotlin 'parameter non-null exception'
This week I found myself confused by an exception when writing a simple extension method on Android’s TextView.
Consider the following extension method:
fun TextView.doSomething() {
}This is about as simple as it could get - it has no parameters and returns nothing. However, when I tried to use it, I found myself with the following exception:
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| Process: com.emmaguy.sampleapp, PID: 3053 | |
| java.lang.RuntimeException: Unable to start activity ComponentInfo{com.emmaguy.sampleapp/com.emmaguy.sampleapp.MainActivity}: java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter $receiver | |
| at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416) | |
| at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) | |
| at android.app.ActivityThread.-wrap11(ActivityThread.java) | |
| at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) | |
| at android.os.Handler.dispatchMessage(Handler.java:102) | |
| at android.os.Looper.loop(Looper.java:148) | |
| at android.app.ActivityThread.main(ActivityThread.java:5417) | |
| at java.lang.reflect.Method.invoke(Native Method) | |
| at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) | |
| at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) | |
| Caused by: java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter $receiver | |
| at com.emmaguy.sampleapp.MainActivityKt.extension(MainActivity.kt) | |
| at com.emmaguy.sampleapp.MainActivity.onCreate(MainActivity.kt:14) | |
| at android.app.Activity.performCreate(Activity.java:6237) | |
| at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107) | |
| at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369) |
Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter $receiver
But that’s weird - it looks like there’s a parameter called $receiver which is null… Except, I don’t have a parameter at all!
Turns out, the TextView I was using was null! The receiver mentioned in the error message refers to the class the extension is called on. 😮