Replace a DCHECK in BVC with an early return.

ios/chrome/browser/ui/browser_view_controller.mm

Index: ios/chrome/browser/ui/browser_view_controller.mm
diff --git a/ios/chrome/browser/ui/browser_view_controller.mm b/ios/chrome/browser/ui/browser_view_controller.mm
index 1beacd5bfcf280a37bc1afa71678e7b0c02942bd..9f094c76cb969094aa005718fd0be43fc4aef212 100644
--- a/ios/chrome/browser/ui/browser_view_controller.mm
+++ b/ios/chrome/browser/ui/browser_view_controller.mm
@@ -4404,7 +4404,12 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
 }
 
 - (void)updateFindBar:(BOOL)initialUpdate shouldFocus:(BOOL)shouldFocus {
-  DCHECK([_model currentTab]);
+  // TODO(crbug.com/731045): This early return temporarily replaces a DCHECK.
+  // For unknown reasons, this DCHECK sometimes was hit in the wild, resulting
+  // in a crash.
+  if (![_model currentTab]) {
+    return;
+  }
   auto* helper = FindTabHelper::FromWebState([_model currentTab].webState);
   if (helper && helper->IsFindUIActive()) {
     if (initialUpdate && !_isOffTheRecord) {