theory,
https://medium.com/@TuiZ/digest-already-in-progress-c0f97a6becfc

solution,
https://code.angularjs.org/1.2.16/docs/error/$rootScope/inprog?p0=$apply
or
https://stackoverflow.com/questions/15112584/how-do-i-use-scope-watch-and-scope-apply-in-angularjs