filemanager: only call callback if it is a function

src/js/filemanager.js

@@ -757,7 +757,7 @@ app.controller('FileManagerController', ['$scope', '$translate', '$timeout', 'Cl
                 $timeout(function () {
                     $scope.textEditor.unsaved = false;
                     $scope.textEditor.busy = false;
-                    if (callback) return callback();
+                    if (typeof callback === 'function') return callback();
                 }, 1000);
             });
         },