[MediaProjection] Fix NPE introduced from deadlock fix

Bug: 293133367
Test: Manual
Change-Id: I9c21acf069c66f1ade22c49b095ad5339fe13eca

services/core/java/com/android/server/media/projection/MediaProjectionManagerService.java

@@ -260,7 +260,9 @@ public final class MediaProjectionManagerService extends SystemService
         }
 
         synchronized (mLock) {
-            mProjectionGrant.stop();
+            if (mProjectionGrant != null) {
+                mProjectionGrant.stop();
+            }
         }
     }