It's a bug in the latest version of UI5 and seem to have fixed and a patch maybe released sooner regarding this.
[FIX] sap/m/BusyDialog: adding a dummy render function to renderer · SAP/openui5@42d76c5 · GitHub
As Parag Jain has stated, the issue can be suppressed by using the previous versions or openui5 beta as of now. If there is any case where you can't bootstrap to other versions, you can do a workaround as below before the BusyDialog is instantiated.
jQuery.sap.declare('sap.m.BusyDialogRenderer');
sap.m.BusyDialogRenderer = {};
sap.m.BusyDialogRenderer.render = function(oRm, oControl) {};