Strange stack at android crash report

I receive some crash reports from android (with java.lang.NullPointerException), but I don't understand what mean __null __ in stacktrace below:

at __null__.formatElapsedTime(MainActivity.java)
at __null__.access$102(MainActivity.java)
at __null__.access$200(MainActivity.java)
at __null__.access$500(MainActivity.java)
at ru.yandex.subbota_job.multiplicationtable.MainActivity.onEnterPressed(MainActivity.java)
at ru.yandex.subbota_job.multiplicationtable.KeyboardFragment.onClick(KeyboardFragment.java)
at android.view.View.performClick(View.java:4463)
at android.view.View$PerformClick.run(View.java:18789)
at android.os.Handler.handleCallback(Handler.java:808)
at android.os.Handler.dispatchMessage(Handler.java:103)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5299)
at java.lang.reflect.Method.invokeNative(Method.java)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645)
at dalvik.system.NativeStart.main(NativeStart.java)

And I don't understand what access$xxx functions is?

Furthermore, formatElapsedTime is not called from onEnterPressed neither directly nor indirectly! That is absolutely! formatElapsedTime is called from timer thread through runOnUiThread. This is very rare bug has occurred for the second time only on two different devices.

Help, please, by any idea! I have a lot of __null__

This was the old behavior when the Firebase stack trace deobfuscator was unable to unambiguously determine which of several methods was the original method for an obfuscated stack frame in a stack trace. This should not happen for future crashes.

Further explanation here.