Crash Reported on Google Play: java.lang.NullPointerException

java.lang.NullPointerException
at org.tomdroid.sync.SyncManager.cancel(SyncManager.java:106)
at org.tomdroid.ui.Tomdroid$15.onCancel(Tomdroid.java:675)
at android.app.Dialog$ListenersHandler.handleMessage(Dialog.java:1050)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:130)
at android.app.ActivityThread.main(ActivityThread.java:3835)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:507)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:864)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:622)
at dalvik.system.NativeStart.main(Native Method)

User message:
synced. put app to background. when opened again it crashed.

Apparently, we are not stopping the synchronisation onDestroy().
this leads to uninitialized variables on next startup.
Should we check this onResume() or onDestroy()?