There is an acknowledged bug in the latest (as of May 2019) update to Mac InDesign CC, 14.0.2. You are most likely to run into it if you are trying to register your License Key, using the Register button in IDImposer. The registration dialog will display, but then both IDImposer and InDesign will lock up when you attempt to dismiss the Registration dialog, requiring you kill InDesign (Command-Option-Esc).
This bug affects many scripts, not just IDImposer, and there is, unfortunately, nothing that I can do to IDImposer to work around the problem.
The only known solution is to back out of the 14.0.2 update, and run 14.0.1, or earlier release of InDesign CC, or InDesign CS6. Hopefully a newer release of InDesign will also include a fix for the problem.
— 30 June 2019 Update —
Adobe just issued a patch to 14.0.2 for this bug. See: https://helpx.adobe.com/indesign/kb/Script-UI-dialog-issue.html
Note that the patch doesn’t change the InDesign version number. In particular, this is NOT a “14.0.3” update.
If you want to learn more about the issue, see this Adobe forum discussion: https://forums.adobe.com/message/11071586#11071586