Proguard: NullPointerException in ArrayAdapter.createViewFromResource without any of my classes in t

My app starts without any problem. But while I log in and the next activity is about to start the app crashes with following Exception:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
    at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:453)
    at android.widget.ArrayAdapter.getView(ArrayAdapter.java:415)
    at android.widget.AbsSpinner.onMeasure(AbsSpinner.java:204)
    at android.widget.Spinner.onMeasure(Spinner.java:602)
    at android.view.View.measure(View.java:23169)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
    at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1187)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:706)
    at android.view.View.measure(View.java:23169)
    at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1168)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:706)
    at android.view.View.measure(View.java:23169)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:825)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
    at android.view.View.measure(View.java:23169)
    at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
    at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
    at android.view.View.measure(View.java:23169)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:825)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
    at android.view.View.measure(View.java:23169)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
    at android.view.View.measure(View.java:23169)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
    at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:461)
    at android.view.View.measure(View.java:23169)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
    at com.android.internal.policy.DecorView.onMeasure(DecorView.java:716)
    at android.view.View.measure(View.java:23169)
    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2718)
    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1572)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1855)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1460)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7183)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:949)
    at android.view.Choreographer.doCallbacks(Choreographer.java:761)
    at android.view.Choreographer.doFrame(Choreographer.java:696)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:935)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

What surprises me is that none of my classes or an external library appears. If I keep my whole package, the next activity starts successfully. But then I don't need to use obfuscation either. Which is strange too: If I keep my whole package it works fine. But if I keep the two sub-packages (package.name.technical & package.name.functional), it won't work.

I keep a lot of View things, which I found in the internet. For e.g.:

-keep class android.widget.AutoCompleteTextView {*;}
-keep class android.widget.Spinner {*;}
-keep class android.widget.ArrayAdapter {*;}
-keep public class * extends android.widget.AdapterView {*;}
-keep public class * extends android.widget.Adapter {*;}
-keep public class * extends android.widget.BaseAdapter {*;}
-keep public class * extends android.view.View {
    public <init>(android.content.Context);
    public <init>(android.content.Context, android.util.AttributeSet);
    public <init>(android.content.Context, android.util.AttributeSet, int);
    public void set*(...);
}

I also keep the activity that should start.

"shrinkResources" is NOT enabled in build.gradle.

Does anyone have any idea why? I'm getting really desperate and don't know what to do.