Fix currentActivity null pointer exception

[AndrewJack]

Seeing a lot of crashes on crashlytics:

Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
       at com.reactNativeQuickActions.AppShortcutsModule.setShortcutItems(AppShortcutsModule.java:102)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.facebook.react.bridge.BaseJavaModule$JavaMethod.invoke(BaseJavaModule.java:368)
       at com.facebook.react.cxxbridge.JavaModuleWrapper.invoke(JavaModuleWrapper.java:138)
       at com.facebook.react.bridge.queue.NativeRunnable.run(NativeRunnable.java)
       at android.os.Handler.handleCallback(Handler.java:751)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at com.facebook.react.bridge.queue.MessageQueueThreadHandler.dispatchMessage(MessageQueueThreadHandler.java:31)
       at android.os.Looper.loop(Looper.java:154)
       at com.facebook.react.bridge.queue.MessageQueueThreadImpl$3.run(MessageQueueThreadImpl.java:208)
       at java.lang.Thread.run(Thread.java:761)

getCurrentActivity() is @Nullable so we should check.

Guessing the method was called at the same time the app was closing.

Thanks in advance @jordanbyron

[AndrewJack]

Hi @jordanbyron do you have time to release this?

This should be the last of the issues 😄

Thanks.

[jordanbyron]

Hey @AndrewJack!

I'll have time on Friday to take a look at this. Could you give me steps to reproduce the error you are fixing so I can verify this PR fixes them.

Thanks!

Jordan

[AndrewJack]

Steps:

1. call setShortcutItems
2. Press back immediately after

Actual: Will crash with exception above
Expected: Doesn't crash if activity has been destroyed.

It's a tough one to reproduce because you have to time it perfectly, but it happens fairy frequently in the wild.