The error is shown in the browser console when clicking on the counters, incidents, etc. on the activities or selecting/deselecting activities.
I could reproduce also the error in Chrome: deps.js?bust=7.6.1:10148:
TypeError: Cannot read property 'prop' of null at Object.positionElements (camunda-cockpit-ui.js?bust=7.6.1:22) at z (camunda-cockpit-ui.js?bust=7.6.1:23) at camunda-cockpit-ui.js?bust=7.6.1:23 at wrappedCallback (deps.js?bust=7.6.1:11723) at deps.js?bust=7.6.1:11809 at Scope.$eval (deps.js?bust=7.6.1:12852) at Scope.$digest (deps.js?bust=7.6.1:12664) at $$childScopeClass.$apply (deps.js?bust=7.6.1:12956) at HTMLSpanElement.m (camunda-cockpit-ui.js?bust=7.6.1:23) at HTMLSpanElement.handle (deps.js?bust=7.6.1:152099)