Exit split if one of the split is empty

IllegalStateException is thrown if one of the splits is empty during
start animaiton. This CL performs the split dismiss action instead
of making the system-ui crash.

Bug: 273426456
Test: make a split with map app. start a full screen on top.
Change-Id: I5d2cae97187bb45df7b550cc99980c1aa792df2d

libs/WindowManager/Shell/src/com/android/wm/shell/splitscreen/StageCoordinator.java

@@ -2321,9 +2321,14 @@ public class StageCoordinator implements SplitLayout.SplitLayoutHandler,
                 }
             }
             if (mMainStage.getChildCount() == 0 || mSideStage.getChildCount() == 0) {
-                // TODO(shell-transitions): Implement a fallback behavior for now.
-                throw new IllegalStateException("Somehow removed the last task in a stage"
-                        + " outside of a proper transition");
+                Log.e(TAG, "Somehow removed the last task in a stage outside of a proper "
+                        + "transition.");
+                final WindowContainerTransaction wct = new WindowContainerTransaction();
+                final int dismissTop = mMainStage.getChildCount() == 0
+                        ? STAGE_TYPE_MAIN : STAGE_TYPE_SIDE;
+                prepareExitSplitScreen(dismissTop, wct);
+                mSplitTransitions.startDismissTransition(wct, this, dismissTop,
+                        EXIT_REASON_UNKNOWN);
                 // This can happen in some pathological cases. For example:
                 // 1. main has 2 tasks [Task A (Single-task), Task B], side has one task [Task C]
                 // 2. Task B closes itself and starts Task A in LAUNCH_ADJACENT at the same time