fixed missing argument in call to popup_messagebox() in particle picking gui frame

[McHaillet]

This is a fix for issue 113. Only added a missing parameter to a function call.

(added by sander): closes #113

[McHaillet]

Can I link a pull request to a specific issue?

[sroet]

add the words "closes #..." to the initial comment and it should automatically close the issue once this gets merged (I will edit your initial comment for now)

[McHaillet]

Nice!