When editing a page's action groups to add/edit an action, Clicking "OK" on the Action group form will throw the error, "Required field missing: Select a tab". This only happens when editing the ActionGroups by going to Page properties > ActionGroups > Clicking elipsis > Adding or Editing an Action Group > Clicking OK.
On the page itself, by going to Edit action groups directly (instead of clicking Properties), the user can add/edit the Action group without error.
After entering design mode, click Edit action groups > Edit an action group, then make the desired changes.
This is the suggested solution for this issue.
Steps to Duplicate
1) Go to a page in CRM > Go into design mode 2) Click Properties > ActionGroups > Click elipsis > Edit an Action group without making changes > Click OK > The error is thrown