Check if target is attached to window before trying to mask it.

app/src/main/java/org/thoughtcrime/securesms/components/MaskView.java

@@ -67,7 +67,7 @@ public class MaskView extends View {
   protected void onDraw(@NonNull Canvas canvas) {
     super.onDraw(canvas);
 
-    if (target == null) {
+    if (target == null || !target.isAttachedToWindow()) {
       return;
     }