How can I debug a packaged app?

My code runs in the normal manner, but the App created with "Package App" does not. It displays crypic error messages, shown below, that don't help me debug. I would guess that, on a first attempt, there might be some additional files that I failed to add manually, but the error messages don't name any such files.

Thanks.

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Error using evalin

Undefined function or variable 'x919119429V01_Pkg_XApp'.

Error in appinstall.internal.runapp>execute (line 116)

out = evalin('caller', [script ';']);

Error in appinstall.internal.runapp>runapp13a (line 95)

outobj = execute(fullfile(appinstalldir, [wrapperfile 'App.m']));

Error in appinstall.internal.runapp>preamble18b (line 60)

appobj = runapp13a(appinstalldir);

Error in appinstall.internal.runapp (line 13)

out = preamble18b(appinstalldir);

Error in matlab.apputil.run (line 45)

appinstall.internal.runapp(appName, appLocation);