Ensure GeckoSession is not null in cleanResources (#3400)

Author: bluemarvin

app/src/common/shared/org/mozilla/vrbrowser/browser/engine/Session.java

@@ -609,7 +609,9 @@ public CompletableFuture<Void> captureBackgroundBitmap(int displayWidth, int dis
 
         Runnable cleanResources = () -> {
             display.surfaceDestroyed();
-            mState.mSession.releaseDisplay(display);
+            if (mState.mSession != null) {
+                mState.mSession.releaseDisplay(display);
+            }
             BitmapCache.getInstance(mContext).releaseCaptureSurface();
         };