Nullpointerexception after memory fill

Question

My app works with different listviews. Acts normally, but when memory fills (during simulation or naturally) and system dumps it, app crashes with nullpointerexception on one of the declared ListViews. Views are declared onCreate()

a_images = (ListView) findViewById(R.id.ascii_images);
a_emotes = (ListView) findViewById(R.id.ascii_emotes);

and then called onSectionAttached()

public void onSectionAttached(int number) {
    switch (number) {
        case 1:
            status = 1;
            mTitle = getString(R.string.title_section1);
            a_images.invalidateViews();
            a_images.refreshDrawableState();

Here's the logcat.

05-21 20:16:54.162 11230-11230/com.joudoviny.jonda.copypasta E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.joudoviny.jonda.copypasta, PID: 11230
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.joudoviny.jonda.copypasta/com.example.jonda.copypasta.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.invalidateViews()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2831)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2906)
    at android.app.ActivityThread.-wrap11(Unknown Source:0)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1605)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:172)
    at android.app.ActivityThread.main(ActivityThread.java:6637)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.invalidateViews()' on a null object reference
    at com.example.jonda.copypasta.MainActivity.onSectionAttached(MainActivity.java:303)
    at com.example.jonda.copypasta.MainActivity$PlaceholderFragment.onAttach(MainActivity.java:530)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1363)
    at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809)
    at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3217)
    at android.support.v4.app.FragmentManagerImpl.dispatchCreate(FragmentManager.java:3166)
    at android.support.v4.app.FragmentController.dispatchCreate(FragmentController.java:181)
    at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:319)
    at android.support.v7.app.AppCompatActivity.onCreate(AppCompatActivity.java:84)
    at com.example.jonda.copypasta.MainActivity.onCreate(MainActivity.java:99)
    at android.app.Activity.performCreate(Activity.java:6975)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2784)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2906) 
    at android.app.ActivityThread.-wrap11(Unknown Source:0) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1605) 
    at android.os.Handler.dispatchMessage(Handler.java:105) 
    at android.os.Looper.loop(Looper.java:172) 
    at android.app.ActivityThread.main(ActivityThread.java:6637) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767) 

This happens whenever I use the ListView, in this case in invalidateViews() method.