Recently ran into an exception logged by glean on samples-glean. I suspect this had to do with old or bad data stored in SharedPreferences since the problem was resolved by uninstalling the app and it's associated data and reinstalling.

Relevant piece of the stack trace here:

2019-04-16 08:45:44.859 23783-23783/org.mozilla.samples.glean D/AndroidRuntime: Shutting down VM
2019-04-16 08:45:44.861 23783-23783/org.mozilla.samples.glean E/AndroidRuntime: FATAL EXCEPTION: main
    Process: org.mozilla.samples.glean, PID: 23783
    java.lang.RuntimeException: Failed to call observer method
        at android.arch.lifecycle.ClassesInfoCache$MethodReference.invokeCallback(ClassesInfoCache.java:225)
        at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeMethodsForEvent(ClassesInfoCache.java:193)
        at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeCallbacks(ClassesInfoCache.java:184)
        at android.arch.lifecycle.ReflectiveGenericLifecycleObserver.onStateChanged(ReflectiveGenericLifecycleObserver.java:36)
        at android.arch.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:354)
        at android.arch.lifecycle.LifecycleRegistry.backwardPass(LifecycleRegistry.java:308)
        at android.arch.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:327)
        at android.arch.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:137)
        at android.arch.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:123)
        at android.arch.lifecycle.ProcessLifecycleOwner.dispatchStopIfNeeded(ProcessLifecycleOwner.java:145)
        at android.arch.lifecycle.ProcessLifecycleOwner.access$100(ProcessLifecycleOwner.java:48)
        at android.arch.lifecycle.ProcessLifecycleOwner$1.run(ProcessLifecycleOwner.java:67)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: org.json.JSONException: No value for test.test_td
        at org.json.JSONObject.get(JSONObject.java:392)
        at mozilla.components.service.glean.storages.StorageEngineManager.separateLabeledAndUnlabeledMetrics(StorageEngineManager.kt:90)
        at mozilla.components.service.glean.storages.StorageEngineManager.collect(StorageEngineManager.kt:114)
        at mozilla.components.service.glean.ping.PingMaker.collect(PingMaker.kt:122)
        at mozilla.components.service.glean.GleanInternalAPI.assembleAndSerializePing$service_glean_debug(Glean.kt:306)
        at mozilla.components.service.glean.GleanInternalAPI.sendPings$service_glean_debug(Glean.kt:284)
        at mozilla.components.service.glean.GleanInternalAPI.handleBackgroundEvent(Glean.kt:259)
        at mozilla.components.service.glean.scheduler.GleanLifecycleObserver.onEnterBackground(GleanLifecycleObserver.kt:26)
        at java.lang.reflect.Method.invoke(Native Method)
        at android.arch.lifecycle.ClassesInfoCache$MethodReference.invokeCallback(ClassesInfoCache.java:215)
        at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeMethodsForEvent(ClassesInfoCache.java:193) 
        at android.arch.lifecycle.ClassesInfoCache$CallbackInfo.invokeCallbacks(ClassesInfoCache.java:184) 
        at android.arch.lifecycle.ReflectiveGenericLifecycleObserver.onStateChanged(ReflectiveGenericLifecycleObserver.java:36) 
        at android.arch.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:354) 
        at android.arch.lifecycle.LifecycleRegistry.backwardPass(LifecycleRegistry.java:308) 
        at android.arch.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:327) 
        at android.arch.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:137) 
        at android.arch.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:123) 
        at android.arch.lifecycle.ProcessLifecycleOwner.dispatchStopIfNeeded(ProcessLifecycleOwner.java:145) 
        at android.arch.lifecycle.ProcessLifecycleOwner.access$100(ProcessLifecycleOwner.java:48) 
        at android.arch.lifecycle.ProcessLifecycleOwner$1.run(ProcessLifecycleOwner.java:67) 
        at android.os.Handler.handleCallback(Handler.java:873) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
2019-04-16 08:45:44.876 23783-23783/org.mozilla.samples.glean I/Process: Sending signal. PID: 23783 SIG: 9

Haven't seen this again