fix: race condition when checking current activity

[zolbooo]

We were experiencing some crashes caused by the NullPointerException raised in the library in production.

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window android.app.Activity.getWindow()' on a null object reference
    at com.reactnativesystemnavigationbar.SystemNavigationBarModule.setNavigationColor(SystemNavigationBarModule.java:1)
    at java.lang.reflect.Method.invoke(Method.java)
    at com.facebook.react.bridge.JavaMethodWrapper.invoke(JavaMethodWrapper.java:18)
    at com.facebook.react.bridge.JavaModuleWrapper.invoke(JavaModuleWrapper.java:13)
    at com.facebook.react.bridge.queue.NativeRunnable.run(NativeRunnable.java)
    at android.os.Handler.handleCallback(Handler.java:938)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at com.facebook.react.bridge.queue.MessageQueueThreadHandler.dispatchMessage(MessageQueueThreadHandler.java:1)
    at android.os.Looper.loopOnce(Looper.java:210)
    at android.os.Looper.loop(Looper.java:299)
    at com.facebook.react.bridge.queue.MessageQueueThreadImpl$4.run(MessageQueueThreadImpl.java:8)
    at java.lang.Thread.run(Thread.java:920)

It appears that Activity was stopped before setNavigationColor call.

- ui.lifecycle {
  screen: MainActivity, 
  state: stopped
}
- ui.lifecycle {
  screen: MainActivity, 
  state: saveInstanceState
}
- NullPointerException: Attempt to invoke virtual method 'android.view.Window android.app.Activity.getWindow()' on a null object reference

There is an issue in the code below:

if (getCurrentActivity() == null) {
         promise.reject("Error: ", "false");
         return;
}
final Window view = getCurrentActivity().getWindow();

Because getCurrentActivity() result is not saved, current activity may be replaced by null after if condition worked, which is exactly what happens before the crash. For example,
Happy path: App working -> setNavigationColor() -> app stops -> if getCurrentActivity() == null is true -> return.
Edge case: App working -> setNavigationColor() -> if getCurrentActivity() == null is FALSE -> app stops -> getCurrentActivity().getWindow() -> NPE.

In the edge case NPE happens because app is already stopped, so that getCurrentActivity() returns null and then getWindow() is called on the null instance.

This PR saves getCurrentActivity() result into the variable, so that Activity.getWindow() never raises a NPE.

[kadiraydinli]

Thanks for PR.