mash: Don't crash when dragging a window to the side.

services/ui/ws/window_manager_state.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/ws/window_manager_state.h"
 
 #include "base/memory/weak_ptr.h"
 #include "services/shell/public/interfaces/connector.mojom.h"
 #include "services/ui/common/event_matcher_util.h"
 #include "services/ui/ws/accelerator.h"
@@ skipping 478 matching lines @@
   WindowTree* tree = nullptr;
   if (in_nonclient_area) {
     tree = window_server()->GetTreeWithId(window->id().client_id);
   } else {
     // If the window is an embed root, forward to the embedded window.
     tree = window_server()->GetTreeWithRoot(window);
     if (!tree)
       tree = window_server()->GetTreeWithId(window->id().client_id);
   }
 
-  const ServerWindow* embed_root =
-      tree->HasRoot(window) ? window : GetEmbedRoot(window);
-  while (tree && tree->embedder_intercepts_events()) {
-    DCHECK(tree->HasRoot(embed_root));
-    tree = window_server()->GetTreeWithId(embed_root->id().client_id);
-    embed_root = GetEmbedRoot(embed_root);
+  if (tree) {
+    const ServerWindow* embed_root =
+        tree->HasRoot(window) ? window : GetEmbedRoot(window);
+    while (tree && tree->embedder_intercepts_events()) {
+      DCHECK(tree->HasRoot(embed_root));
+      tree = window_server()->GetTreeWithId(embed_root->id().client_id);
+      embed_root = GetEmbedRoot(embed_root);
+    }
   }
 
   if (!tree) {
     DCHECK(in_nonclient_area);
     tree = window_tree_;
   }
   return tree->id();
 }
 
 ServerWindow* WindowManagerState::GetRootWindowContaining(
     const gfx::Point& location) {
   if (display_manager()->displays().empty())
     return nullptr;
 
   // TODO(sky): this isn't right. To correctly implement need bounds of
   // Display, which we aren't tracking yet. For now, use the first display.
   Display* display = *(display_manager()->displays().begin());
   WindowManagerDisplayRoot* display_root =
       display->GetWindowManagerDisplayRootForUser(user_id());
   return display_root ? display_root->root() : nullptr;
 }
 
 void WindowManagerState::OnEventTargetNotFound(const ui::Event& event) {
   window_server()->SendToEventObservers(event, user_id(),
                                         nullptr /* ignore_tree */);
 }
 
 }  // namespace ws
 }  // namespace ui