Fix Android modal crash: android.view.WindowManager$BadTokenException

Summary:
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Fixes a crash with the Android Dialog. This occurs when the Activity is finishing or has been removed and React Native requests the modal to be shown.

Stacktrace here:
```
Caused by android.view.WindowManager$BadTokenException: Unable to add window -- token android.os.BinderProxy@f125ea3 is not valid; is your activity running?
       at android.view.ViewRootImpl.setView(ViewRootImpl.java:683)
       at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:319)
       at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:85)
       at android.app.Dialog.show(Dialog.java:326)
       at com.facebook.react.views.modal.ReactModalHostView.showOrUpdate(ReactModalHostView.java:256)
       at com.facebook.react.views.modal.ReactModalHostManager.onAfterUpdateTransaction(ReactModalHostManager.java:107)
       at com.facebook.react.views.modal.ReactModalHostManager.onAfterUpdateTransaction(ReactModalHostManager.java:28)
       at com.facebook.react.uimanager.ViewManager.updateProperties(ViewManager.java:35)
       at com.facebook.react.uimanager.NativeViewHierarchyManager.createView(NativeViewHierarchyManager.java:233)
       at com.facebook.react.uimanager.UIViewOperationQueue$CreateViewOperation.execute(UIViewOperationQueue.java:153)
       at com.facebook.react.uimanager.UIViewOperationQueue$1.run(UIViewOperationQueue.java:816)
       at com.facebook.react.uimanager.UIViewOperationQueue.flushPendingBatches(UIViewOperationQueue.java:929)
       at com.facebook.react.uimanager.UIViewOperationQueue.access$2100(UIViewOperationQueue.java:47)
       at com.facebook.react.uimanager.UIViewOperationQueue$2.runGuarded(UIViewOperationQueue.java:887)
       at com.facebook.react.bridge.GuardedRunnable.run(GuardedRunnable.java:21)
       at android.os.Handler.handleCallback(Handler.java:815)
       at android.os.Handler.dispatchMessage(Handler.java:104)
       at android.os.Looper.loop(Looper.java:207)
       at android.app.ActivityThread.main(ActivityThread.java:5728)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)
```

Related issues: #18570 & #18634

mdvacca added to this file in e5c2a66897
Doesn't look like the issue was introduced in the above change, but it masks the issue. The wrapping if block should not allow a null activity to display a dialog.

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Run sample app on Android and display dialog when app is about to close.

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[ANDROID] [BUGFIX] [Modal] - Fix Android BadTokenException crash

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Closes https://github.com/facebook/react-native/pull/18996

Differential Revision: D7938147

Pulled By: hramos

fbshipit-source-id: f4ffd1746304d6184d727339072a7e926ffdaf39

ReactAndroid/src/main/java/com/facebook/react/views/modal/ReactModalHostView.java

@@ -250,7 +250,7 @@ public class ReactModalHostView extends ViewGroup implements LifecycleEventListe
     if (mHardwareAccelerated) {
       mDialog.getWindow().addFlags(WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED);
     }
-    if (currentActivity == null || !currentActivity.isFinishing()) {
+    if (currentActivity != null && !currentActivity.isFinishing()) {
       mDialog.show();
     }
   }