Do not crash when the main activity is unavailable

The React Native application context has a method for obtaining the current Android activity, but this sometimes returns null. We believe this happens due to a race condition between the Javascript engine and the native code, but we aren't completely sure. What we do know is that this situation happens in the wild, since it shows up in our Bugsnag reports.

The fix is the leave the BlurView native component uninitialized in these cases.This prevents it from rendering anything, but that's preferable to crashing the whole app.

android/src/main/java/com/cmcewen/blurview/BlurViewManager.java

@@ -32,14 +32,17 @@ class BlurViewManager extends ViewGroupManager<BlurView> {
     @Override
     public @Nonnull BlurView createViewInstance(@Nonnull ThemedReactContext ctx) {
         BlurView blurView = new BlurView(ctx);
-        View decorView = Objects.requireNonNull(ctx.getCurrentActivity()).getWindow().getDecorView();
-        ViewGroup rootView = decorView.findViewById(android.R.id.content);
-        Drawable windowBackground = decorView.getBackground();
-        blurView.setupWith(rootView)
-            .setFrameClearDrawable(windowBackground)
-            .setBlurAlgorithm(new RenderScriptBlur(ctx))
-            .setBlurRadius(defaultRadius)
-            .setHasFixedTransformationMatrix(false);
+        Activity currentActivity = ctx.getCurrentActivity();
+        if (currentActivity != null) {
+            View decorView = currentActivity.getWindow().getDecorView();
+            ViewGroup rootView = decorView.findViewById(android.R.id.content);
+            Drawable windowBackground = decorView.getBackground();
+            blurView.setupWith(rootView)
+                .setFrameClearDrawable(windowBackground)
+                .setBlurAlgorithm(new RenderScriptBlur(ctx))
+                .setBlurRadius(defaultRadius)
+                .setHasFixedTransformationMatrix(false);
+        }
         return blurView;
     }