From f67310964df72996863f772fb1bbf9aeb9ee3744 Mon Sep 17 00:00:00 2001
From: timeless <timeless@mozdev.org>
Date: Thu, 3 Dec 2009 01:37:33 -0600
Subject: [PATCH] Bug 530322 - crash [@
 nsWindowWatcher::SizeOpenedDocShellItem(nsIDocShellTreeItem*, nsIDOMWindow*,
 SizeSpec const&)] [r=bzbarsky]

---
 .../components/windowwatcher/src/nsWindowWatcher.cpp     | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/embedding/components/windowwatcher/src/nsWindowWatcher.cpp b/embedding/components/windowwatcher/src/nsWindowWatcher.cpp
index cec251016f7..6ea021be037 100644
--- a/embedding/components/windowwatcher/src/nsWindowWatcher.cpp
+++ b/embedding/components/windowwatcher/src/nsWindowWatcher.cpp
@@ -1921,7 +1921,14 @@ nsWindowWatcher::SizeOpenedDocShellItem(nsIDocShellTreeItem *aDocShellItem,
   }
   if (mainWidget) {
     nsCOMPtr<nsIDeviceContext> ctx = mainWidget->GetDeviceContext();
-    devPixelsPerCSSPixel = float(ctx->AppUnitsPerCSSPixel()) / ctx->AppUnitsPerDevPixel();
+    /* we might be called by an extension after mainWidget::OnDestroy() */
+    if (ctx) {
+      PRInt32 unitsPerDevPixel = ctx->AppUnitsPerDevPixel();
+      if (unitsPerDevPixel) {
+        devPixelsPerCSSPixel = float(ctx->AppUnitsPerCSSPixel()) /
+                                     unitsPerDevPixel;
+      }
+    }
   }
 
   /* The current position and size will be unchanged if not specified