You do not say which version and version of VS your client is using, but at least in VS 2005 and VS 2010 open Tools-> Options-> Debugging-> General, there is a list of settings, scroll down, find "Warn if there are no characters when launch (only for relatives) "and uncheck.
I believe that this will accomplish what you are looking for since the dialog will not appear until / until the debugger stops in your code. If it stops (or hits a breakpoint) in the client plugin, it should be able to find characters, and I think this will not cause a popup.
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