[CAT-1095] FormulaEditorComputeDialog closes on inside click.

Fixes CAT-1095! ;)

Jirra issue: https://jira.catrob.at/browse/CAT-1095
Jenkins testrun: https://jenkins.catrob.at/view/All-Categories/view/Catroid-multi-job/job/Catroid-Multi-Job-Custom-Branch/2318/