I have a working app and everything was working well, but when I updated it with MoPub app and generated a new signed APK and uploaded it to the play store some users reported a crash while it works well on other users' devices.
I tested the app a lot on different devices and finally found a device that it crashed on.
I debugged the app and found this error code in the android monitor
06-18 16:17:51.665 17951-17951/akhbar4all.com.akhbar4all W/dalvikvm: VFY: unable to find class referenced in signature (Lcom/google/firebase/auth/FirebaseUser;) 06-18 16:17:51.665 17951-17951/akhbar4all.com.akhbar4all E/dalvikvm: Could not find class 'com.google.firebase.FirebaseApp$zza', referenced from method com.google.firebase.FirebaseApp.zza 06-18 16:17:51.665 17951-17951/akhbar4all.com.akhbar4all W/dalvikvm: VFY: unable to resolve check-cast 9003 (Lcom/google/firebase/FirebaseApp$zza;) in Lcom/google/firebase/FirebaseApp; 06-18 16:17:51.665 17951-17951/akhbar4all.com.akhbar4all D/dalvikvm: VFY: replacing opcode 0x1f at 0x0019 06-18 16:17:51.665 17951-17951/akhbar4all.com.akhbar4all D/dalvikvm: DexOpt: unable to opt direct call 0xec65 at 0x0b in Lcom/google/firebase/FirebaseApp;.getToken 06-18 16:17:51.665 17951-17951/akhbar4all.com.akhbar4all D/AndroidRuntime: Shutting down VM 06-18 16:17:51.665 17951-17951/akhbar4all.com.akhbar4all W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x41bdc700) 06-18 16:17:51.685 17951-17951/akhbar4all.com.akhbar4all E/AndroidRuntime: FATAL EXCEPTION: main java.lang.NoClassDefFoundError: com.google.firebase.FirebaseOptions at com.google.firebase.FirebaseApp.zzbu(Unknown Source) at com.google.firebase.provider.FirebaseInitProvider.onCreate(Unknown Source) at android.content.ContentProvider.attachInfo(ContentProvider.java:1214) at android.content.ContentProvider.attachInfo(ContentProvider.java:1189) at com.google.firebase.provider.FirebaseInitProvider.attachInfo(Unknown Source) at android.app.ActivityThread.installProvider(ActivityThread.java:5119) at android.app.ActivityThread.installContentProviders(ActivityThread.java:4725) at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4665) at android.app.ActivityThread.access$1400(ActivityThread.java:159) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1376) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:176) at android.app.ActivityThread.main(ActivityThread.java:5419) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:525) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862) at dalvik.system.NativeStart.main(Native Method)
But I was shocked because I didn't use any Firebase api so why is this happening and what is the solution? I'd be happy for any help from someone.