Fix NPE in isTrimmable

Previous CL[1] makes the root home task nullable, which causes NPE
in RecentTask#isTrimmable. This CL adds null checks on the method and
other callers which don't have checks, and also marks
TaskDisplayArea#getRootHomeTask as @Nullable.

[1]: 3388bd2a0b

Test: atest RecentTasks RecentsAnimationControllerTest
Test: atest TaskDisplayAreaTests WindowStateTests
fixes: 155100679
Change-Id: I8a59fbcb99810172a3b3192917d24e5122a02476

services/core/java/com/android/server/wm/RecentTasks.java

@@ -1387,9 +1387,7 @@ class RecentTasks {
         return false;
     }
 
-    /**
-     * @return whether the given task can be trimmed even if it is outside the visible range.
-     */
+    /** @return whether the given task can be trimmed even if it is outside the visible range. */
     protected boolean isTrimmable(Task task) {
         final ActivityStack stack = task.getStack();
 
@@ -1404,9 +1402,13 @@ class RecentTasks {
             return false;
         }
 
+        final ActivityStack rootHomeTask = stack.getDisplayArea().getRootHomeTask();
+        // Home stack does not exist. Don't trim the task.
+        if (rootHomeTask == null) {
+            return false;
+        }
         // Trim tasks that are behind the home task.
-        final TaskDisplayArea taskDisplayArea = stack.getDisplayArea();
-        return task.compareTo(taskDisplayArea.getRootHomeTask()) < 0;
+        return task.compareTo(rootHomeTask) < 0;
     }
 
     /** Remove the tasks that user may not be able to return. */

services/core/java/com/android/server/wm/RecentsAnimationController.java

@@ -406,8 +406,9 @@ public class RecentsAnimationController implements DeathRecipient {
         }
 
         // Save the minimized home height
-        mMinimizedHomeBounds = mDisplayContent.getDefaultTaskDisplayArea().getRootHomeTask()
-                .getBounds();
+        final ActivityStack rootHomeTask =
+                mDisplayContent.getDefaultTaskDisplayArea().getRootHomeTask();
+        mMinimizedHomeBounds = rootHomeTask != null ? rootHomeTask.getBounds() : null;
 
         mService.mWindowPlacerLocked.performSurfacePlacement();
 

services/core/java/com/android/server/wm/TaskDisplayArea.java

@@ -195,7 +195,7 @@ final class TaskDisplayArea extends DisplayArea<ActivityStack> {
         return mChildren.indexOf(stack);
     }
 
-    ActivityStack getRootHomeTask() {
+    @Nullable ActivityStack getRootHomeTask() {
         return mRootHomeTask;
     }
 

services/core/java/com/android/server/wm/WindowState.java

@@ -1509,7 +1509,7 @@ class WindowState extends WindowContainer<WindowState> implements WindowManagerP
         return mActivityRecord != null ? mActivityRecord.getTask() : null;
     }
 
-    ActivityStack getRootTask() {
+    @Nullable ActivityStack getRootTask() {
         final Task task = getTask();
         if (task != null) {
             return (ActivityStack) task.getRootTask();
@@ -2532,7 +2532,7 @@ class WindowState extends WindowContainer<WindowState> implements WindowManagerP
             final Task task = getTask();
             if (task != null) {
                 task.getDimBounds(mTmpRect);
-            } else {
+            } else if (getRootTask() != null) {
                 getRootTask().getDimBounds(mTmpRect);
             }
         }