Add null check before detach content view from dragView at onDropComplete

Test: Drop a conversation widget on a configuration button. No crash
      is observed.
Fix: 191762933
Change-Id: I483c6f4dbc9fc6c096383682284a4c854707f583
diff --git a/src/com/android/launcher3/Workspace.java b/src/com/android/launcher3/Workspace.java
index 303bb01..f28f54a 100644
--- a/src/com/android/launcher3/Workspace.java
+++ b/src/com/android/launcher3/Workspace.java
@@ -2792,7 +2792,7 @@
             }
         } else if (mDragInfo != null) {
             // When drag is cancelled, reattach content view back to its original parent.
-            if (mDragInfo.cell instanceof LauncherAppWidgetHostView) {
+            if (mDragInfo.cell instanceof LauncherAppWidgetHostView && d.dragView != null) {
                 d.dragView.detachContentView(/* reattachToPreviousParent= */ true);
             }
             final CellLayout cellLayout = mLauncher.getCellLayout(