Fix possible NPE in MenuItemImpl.toString()

BUG: 24474852
Change-Id: If3089d5f140d753145f5b75b50b0f9402c5756ba
diff --git a/v7/appcompat/src/android/support/v7/internal/view/menu/MenuItemImpl.java b/v7/appcompat/src/android/support/v7/internal/view/menu/MenuItemImpl.java
index ffddb31..ed2afc0 100644
--- a/v7/appcompat/src/android/support/v7/internal/view/menu/MenuItemImpl.java
+++ b/v7/appcompat/src/android/support/v7/internal/view/menu/MenuItemImpl.java
@@ -538,7 +538,7 @@
 
     @Override
     public String toString() {
-        return mTitle.toString();
+        return mTitle != null ? mTitle.toString() : null;
     }
 
     void setMenuInfo(ContextMenuInfo menuInfo) {