Fix invisible issue with double-click docking

It generated silent errors because I forgot to add this check, only if
you double-clicked on a window on which that isn't supposed to do
anything. So yeah, that's fixed. Whatever, though.

src/$Window.js

@@ -64,7 +64,9 @@ function $Window($component){
 		$G.off("mousemove", drag);
 	});
 	$w.$titlebar.on("dblclick", function(e){
-		$component.dock();
+		if($component){
+			$component.dock();
+		}
 	});
 	
 	$w.$Button = function(text, handler){