Fixes a NullPointerException that crashes the app (#236)

* Fix NullPointerException

* Fix syntax error

Author: chintanparikh

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

@@ -50,10 +50,12 @@ public void setDownsampleFactor(BlurringView view, int factor) {
 
     @ReactProp(name = "viewRef")
     public void setViewRef(BlurringView view, int viewRef) {
-        View viewToBlur = context.getCurrentActivity().findViewById(viewRef);
+        if (context != null && context.getCurrentActivity() != null) {
+          View viewToBlur = context.getCurrentActivity().findViewById(viewRef);
 
-        if (viewToBlur != null) {
-            view.setBlurredView(viewToBlur);
+          if (viewToBlur != null) {
+              view.setBlurredView(viewToBlur);
+          }
         }
     }
 }