Android Studio: initializing shared preferences crashing activity launch - android-studio

Question

My android app is crashing at activity launch from the main activity, but only when I initialize a SharedPreferences object.
In Main Activity:
package my.investing.investingtestapp
import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
btnPlay.setOnClickListener {
// Handler code here.
val I = Intent(this, PlayActivity::class.java)
startActivity(I);
}
}
}
In Play Activity:
package my.company.example
import android.content.Context
import android.content.SharedPreferences
import android.content.SharedPreferences.Editor
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
class PlayActivity : AppCompatActivity() {
val prefs: SharedPreferences? by lazy { this.getPreferences(Context.MODE_PRIVATE) }
val editor: Editor = prefs!!.edit()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_play2)
}
}
This is true also for:
val prefs = this.getPreferences(Context.MODE_PRIVATE)
error:
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{my.company.example/my.company.example.PlayActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3365)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:149)
at android.app.Activity.getLocalClassName(Activity.java:6738)
at android.app.Activity.getPreferences(Activity.java:6782)
at my.company.example.PlayActivity$prefs$2.invoke(PlayActivity.kt:39)
at my.company.example.PlayActivity$prefs$2.invoke(PlayActivity.kt:37)
at kotlin.SynchronizedLazyImpl.getValue(LazyJVM.kt:74)
at my.company.example.PlayActivity.getPrefs(Unknown Source:2)
at my.company.example.PlayActivity.(PlayActivity.kt:40)
at java.lang.Class.newInstance(Native Method)
at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
at android.app.Instrumentation.newActivity(Instrumentation.java:1253)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3353)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
at android.os.Handler.dispatchMessage(Handler.java:106) 
at android.os.Looper.loop(Looper.java:223) 
at android.app.ActivityThread.main(ActivityThread.java:7656) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

Answer 1

Specifically, it's this:
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.String android.content.Context.getPackageName()' on a null object
reference at
android.content.ContextWrapper.getPackageName(ContextWrapper.java:149) at
android.app.Activity.getLocalClassName(Activity.java:6738) at
android.app.Activity.getPreferences(Activity.java:6782) at
my.company.example.PlayActivity$prefs$2.invoke(PlayActivity.kt:39) at
It's crashing because you're trying to call getPackageName() on a null Context, which is happening through a getPreferences() call caused by your prefs object at line 39 of PlayActivity.kt.
Those member functions are initialised when your Activity is constructed, but at that point the Activity doesn't have a Context yet - that happens later in the lifecycle, and it's ready by the time onCreate is called. So some things have to be initialised later (lateinit is useful for things like this)
Your lazy delegate would work normally - by the time it's accessed by something, the Activity would have its Context. But you're accessing it in the next line when you initialise editor, so it's always running at construction anyway and there's no point making it lazy.
You probably shouldn't hold an editor reference like that anyway, especially with things like KTX Core that let you edit, set and commit all in one shot:
sharedPreferences.edit { putBoolean("key", value)
Definitely don't use !! on a thing that definitely can be null sometimes, you'll only end up regretting it

Answer 2

You cannot call methods that you inherit from Activity until inside of the onCreate() method. Until then, things are not set up yet.
So intialize your objects in onCreate() method.

Answer 3

Try to editor = prefs?.edit() into oncreate after setcontentview.
Btw operator !! Is not safety, because it could throw KotlinNullPointerException