java.lang.NullPointerException on version upgrade

My application throws an exception when I install the apk on an existing version.

These are the steps to reproduce my error:

1. Install application (i.e. v1) and run it (no error found)
2. Install the same apk or a new one (i.e. v2) with a minor update
3. A NullReferenceException is thrown
4. Install the same apk of #2
5. Everything works fine

This is the exception:

java.lang.RuntimeException: Unable to start activity
ComponentInfo{com.empystudio.cashflow/com.empystudio.cashflow.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
at android.app.ActivityThread.access$600(ActivityThread.java:130)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.empystudio.cashflow.MainActivity.onCreate(MainActivity.java:66)
at android.app.Activity.performCreate(Activity.java:5008)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
... 11 more

This is the code of MainActivity

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    documentsView = (ImageView) findViewById(R.id.documents_view);
    backupView