prevent crash with pin protected user and taskview

add forgotten null check

Bug: 265981088
Test: unit tests, CTS tests, manual tests on target
Change-Id: I16904357b28cbf73f19291010427854a45af6adc

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

@@ -1233,7 +1233,7 @@ final class TaskDisplayArea extends DisplayArea<WindowContainer> {
 
         // Clear last paused activity if focused root task changed while sleeping, so that the
         // top activity of current focused task can be resumed.
-        if (mDisplayContent.isSleeping()) {
+        if (mDisplayContent.isSleeping() && currentFocusedTask != null) {
             currentFocusedTask.clearLastPausedActivity();
         }