closes #666, the issue of the beast

src/websockets.js

@@ -593,7 +593,9 @@ websockets.init = function(io) {
 
 		socket.on('api:categories.get', function(callback) {
 			categories.getAllCategories(0, function(err, categories) {
-				callback(categories);
+				if(callback) {
+					callback(categories);
+				}
 			});
 		});