resolved conflicts for merge of 4849aa86 to master

Change-Id: I7ec55bdb7a3a1618f33dfdb3b19b2bd201789677
diff --git a/services/core/java/com/android/server/wm/WindowState.java b/services/core/java/com/android/server/wm/WindowState.java
index 97178bd..7003c8c 100644
--- a/services/core/java/com/android/server/wm/WindowState.java
+++ b/services/core/java/com/android/server/wm/WindowState.java
@@ -728,8 +728,11 @@
     }
 
     public DisplayContent getDisplayContent() {
-        return mAppToken == null || mNotOnAppsDisplay ?
-                mDisplayContent : getStack().getDisplayContent();
+        if (mAppToken == null || mNotOnAppsDisplay) {
+            return mDisplayContent;
+        }
+        final TaskStack stack = getStack();
+        return stack == null ? mDisplayContent : stack.getDisplayContent();
     }
 
     public int getDisplayId() {