I got an error like this when Add Item 
Loading takes very long and doesn’t stop
And an error appears like this when inspect elements.
Failed to load resource: the server responded with a status of 500 (Internal Server Error)
Uncaught TypeError: this.mask.addClass is not a function
at S.onShow (modx.window.js:60)
at S.afterShow (ext-all.js:21)
at S.show (ext-all.js:21)
at Object.show (ext-all.js:21)
at MODx.onAjaxException (modx.js?v=e2980a29:122)
at h.Event.fire (ext-all.js:21)
at Ext.data.Connection.fireEvent (ext-all.js:21)
at Ext.data.Connection.handleFailure (ext-all.js:21)
at f (ext-base.js:21)
at m (ext-base.js:21)
Can anyone help me?