Fix a potential NPE

I've never been able to reproduce this and the stack traces I have
of it don't match existing code, but looking at the code paths I think
this could still happen, there are a couple of places where the
mMagnetizedBubbleDraggingOut gets nulled out (e.g. on child removed)
I've tried to repro by:
 - canceling the bubbles in the middle of dragging to dismiss
 - trying to snap the bubble back and then drag it again to dismiss
   (snap back also nulls it)

I see a recent stack of it in pitot so I think it's worth adding the
null check.

Test: atest ExpandedAnimationControllerTest
Bug: 201866808
Change-Id: Ib9425a2b63410c95fddaaea59761dfd18557ed54

libs/WindowManager/Shell/src/com/android/wm/shell/bubbles/animation/ExpandedAnimationController.java

@@ -346,6 +346,9 @@ public class ExpandedAnimationController
      * bubble is dragged back into the row.
      */
     public void dragBubbleOut(View bubbleView, float x, float y) {
+        if (mMagnetizedBubbleDraggingOut == null) {
+            return;
+        }
         if (mSpringToTouchOnNextMotionEvent) {
             springBubbleTo(mMagnetizedBubbleDraggingOut.getUnderlyingObject(), x, y);
             mSpringToTouchOnNextMotionEvent = false;

libs/WindowManager/Shell/tests/unittest/src/com/android/wm/shell/bubbles/animation/ExpandedAnimationControllerTest.java

@@ -59,19 +59,21 @@ public class ExpandedAnimationControllerTest extends PhysicsAnimationLayoutTestC
     private int mStackOffset;
     private PointF mExpansionPoint;
     private BubblePositioner mPositioner;
-    private BubbleStackView.StackViewState mStackViewState;
+    private BubbleStackView.StackViewState mStackViewState = new BubbleStackView.StackViewState();
 
     @SuppressLint("VisibleForTests")
     @Before
     public void setUp() throws Exception {
         super.setUp();
 
-        BubbleStackView stackView = mock(BubbleStackView.class);
-        when(stackView.getState()).thenReturn(getStackViewState());
         mPositioner = new BubblePositioner(getContext(), mock(WindowManager.class));
         mPositioner.updateInternal(Configuration.ORIENTATION_PORTRAIT,
                 Insets.of(0, 0, 0, 0),
                 new Rect(0, 0, mDisplayWidth, mDisplayHeight));
+
+        BubbleStackView stackView = mock(BubbleStackView.class);
+        when(stackView.getState()).thenReturn(getStackViewState());
+
         mExpandedController = new ExpandedAnimationController(mPositioner,
                 mOnBubbleAnimatedOutAction,
                 stackView);
@@ -135,6 +137,12 @@ public class ExpandedAnimationControllerTest extends PhysicsAnimationLayoutTestC
         testBubblesInCorrectExpandedPositions();
     }
 
+    @Test
+    public void testDragBubbleOutDoesntNPE() throws InterruptedException {
+        mExpandedController.onGestureFinished();
+        mExpandedController.dragBubbleOut(mViews.get(0), 1, 1);
+    }
+
     /** Expand the stack and wait for animations to finish. */
     private void expand() throws InterruptedException {
         mExpandedController.expandFromStack(mock(Runnable.class));