This is due to a recently reported bug with Show Custom Dialog and the latest release of FileMaker Go. For More Information see: Known Buglist for Filemaker GO
This is one of many acknowledged bugs that can be found in the Known Buglist for Filemaker GO here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
The first link includes a description of two work arounds that you can try.