Add null check before shutting down executor in ScannerActivity

app/src/main/java/com/beemdevelopment/aegis/ui/ScannerActivity.java

@@ -89,7 +89,9 @@ public class ScannerActivity extends AegisActivity implements QrCodeAnalyzer.Lis
 
     @Override
     protected void onDestroy() {
+        if (_executor != null) {
             _executor.shutdownNow();
+        }
         super.onDestroy();
     }