I would suggest getting assistance from our Adobe Developer Support group for this inquiry (this group supports questions related to the Adobe Drive SDK). Support from this group is available via membership to the Adobe Technology Partner Program or via an Adobe Developer Support single support incident or bundle of incidents: http://www.adobe.com/support/programs/developer/
I have exactly the samme issue. Step by step, i has the same situation and result than Juibra (except that I use Drive 5 instead of 4.2, but the rest is the same). Did you (or someone) found any solution for this problem, please?
I found where the problem was (at least in my case).
To go to the point: go in the "properties" of your flex project. In the menu, go in "Flex Build Path" and then in "Library Path". Then, the option "Framework linkage" is with the default value in a new flex project.
So here is the trick : with Flex 3 (so Adobe Drive 3), the default value is "merged into code". Bug in the Flex 4 (so Adobe Drive 5), the default value is "Runtime Shared Library".
And because the module is ran independently (with no link with the flex code that load your module), the flash player crashes because it doesn't find your libraries (like ICustomCheckinUI).
I discovered this problem running the Custom UI from the windows explorer (not with Bridge) and using the Flash Player Debug version, and watching the error log files of the Flash Player.
So in the end, when I checked "merged into code" in the Framework Linkage option, all my dependencies of my flex module were merged into my final module swf, and no more flash crash.
Sorry for my bad english and hope it helped.
(edit : screenshot at http://www.hostingpics.net/viewer.php?id=276932flexbuildpath.png )