This error originates at the clientside, in the browser, which is where mxui.js lives. Therefor the error is certainly not caused by a microflow.
“Cannot read properties of null (reading 'getConstraints') “ is caused by a statement like somevariable.getconstraints where somevariable is null. So programmer error in mxui, so inform Mendix since they will need to solve. But you also need to be able to reproduce the error, so you will have to do some investigation to find the cause. To do so, commit your changes, then start removing any suspect element on your page. Start with anything that does front-end tricks. An html-widgets, any other widget. Just keep deleting stuff, run locally, see if it is gone, try again.