Fixing NPE when checking locked state.

- Regression from change 9f4ab32b1c.

Change-Id: I31ada9f4c051cf0aaa9a7bfa54c1b9bda1c64c16
Fixes: b/33738356
Test: Dock an app repeatedly

packages/SystemUI/src/com/android/systemui/recents/views/TaskViewThumbnail.java

@@ -136,7 +136,7 @@ public class TaskViewThumbnail extends View {
         int thumbnailHeight = Math.min(viewHeight,
                 (int) (mThumbnailRect.height() * mThumbnailScale));
 
-        if (mTask.isLocked) {
+        if (mTask != null && mTask.isLocked) {
             canvas.drawRoundRect(0, 0, viewWidth, viewHeight, mCornerRadius, mCornerRadius,
                     mLockedPaint);
         } else if (mBitmapShader != null && thumbnailWidth > 0 && thumbnailHeight > 0) {