If you launch the Plastic installer when an application is running which needs to be closed, you are presented with a list of the applications which need to be closed, and three buttons:
Refresh List - Continue - Cancel
... where Continue is grayed out ...

In order to proceed from that point onward, the user needs to first close the applications, then click on Refresh List, then click on Continue.

You could simplify this by combining the "Refresh List" and "Continue" operations into a single "Refresh" operation:

Remove the "Refresh List" button. Replace the "Continue" button with a "Refresh" button.

This "Refresh" button will refresh the list of apps that need to be closed, and - if the list is empty - automatically proceeds to the next stage in the installer.