Null checks to prevent occasional crash

chrome/browser/ui/browser_list.cc

Index: chrome/browser/ui/browser_list.cc
diff --git a/chrome/browser/ui/browser_list.cc b/chrome/browser/ui/browser_list.cc
index 851a9f8ca791e33639009faadca239dc3ef2017c..cd2a36b45c6308e8e1949f62530898526a4540ea 100644
--- a/chrome/browser/ui/browser_list.cc
+++ b/chrome/browser/ui/browser_list.cc
@@ -151,6 +151,9 @@ void BrowserList::TryToCloseBrowserList(const BrowserVector& browsers_to_close,
     const base::FilePath& profile_path) {
   for (BrowserVector::const_iterator it = browsers_to_close.begin();
        it != browsers_to_close.end(); ++it) {
+    // TODO(mlerman): crbug.com/423229, if we can determine why window() is
+    // false we can better solve the underlying issue here.
+    CHECK((*it)->window());
     if ((*it)->CallBeforeUnloadHandlers(
             base::Bind(&BrowserList::PostBeforeUnloadHandlers,
                        browsers_to_close,
@@ -163,8 +166,12 @@ void BrowserList::TryToCloseBrowserList(const BrowserVector& browsers_to_close,
   on_close_success.Run(profile_path);
 
   for (BrowserVector::const_iterator it = browsers_to_close.begin();
-       it != browsers_to_close.end(); ++it)
+       it != browsers_to_close.end(); ++it) {
+    // TODO(mlerman): If reproducible, determine cause if window() being null.
+    CHECK((*it));
+    CHECK((*it)->window());
     (*it)->window()->Close();
+  }
 }
 
 // static