Only set watchers on custom fields for custom field columns

app/assets/javascripts/angular/directives/timelines/timeline-column-name-directive.js

@@ -9,16 +9,16 @@ angular.module('openproject.timelines.directives')
       localePrefix: '@'
     },
     link: function(scope, element) {
-      scope.$watch('customFields', function(){
       if (CustomFieldHelper.isCustomFieldKey(scope.columnName)) {
+        scope.$watch('customFields', function(){
           var customFieldId = CustomFieldHelper.getCustomFieldId(scope.columnName);
           if (scope.customFields && scope.customFields[customFieldId]) {
             element.html(scope.customFields[customFieldId].name);
           }
+        });
       } else {
         element.html(I18n.t(scope.localePrefix + '.' + scope.columnName));
       }
-      });
 
     }
   };