Fixes to get view_manager_lib_unittests not crash

mojo/examples/window_manager/window_manager.cc

Index: mojo/examples/window_manager/window_manager.cc
diff --git a/mojo/examples/window_manager/window_manager.cc b/mojo/examples/window_manager/window_manager.cc
index 0771cdc442e07f209ae8036a598f0707205b3d1e..9abc98922b1b6527bda6841cdc18e13f644034b9 100644
--- a/mojo/examples/window_manager/window_manager.cc
+++ b/mojo/examples/window_manager/window_manager.cc
@@ -265,7 +265,10 @@ class WindowManager
         app_(NULL) {}
 
   virtual ~WindowManager() {
-    window_manager_app_->host()->window()->RemovePreTargetHandler(this);
+    // host() may be destroyed by the time we get here.
+    // TODO: figure out a way to always cleanly remove handler.
+    if (window_manager_app_->host())
+      window_manager_app_->host()->window()->RemovePreTargetHandler(this);
   }
 
   void CloseWindow(Id view_id) {