GetAppData Error when trying to close an installed MATLAB app that I have built and packaged

Hi!

I am having problems when attempting to close an INSTALLED MATLAB App, that I built and packaged myself.

The app closes correctly with no errors when run normally in MATLAB and as a standalone app using runtime. However, after successfully installing and running the app from 'MATLAB Apps', when you attempt to close the app, it throws out the following error:

Error using getappdata
Value must be a handle.

Error in BeamDeflectionLabApp/attachOncleanupToFigure (line 109)
            appdata = getappdata(fig);

Error in BeamDeflectionLabApp/startApp (line 94)
                obj.attachOncleanupToFigure(appdesigner.internal.service.AppManagementService.getFigure(obj.AppHandle));

Error in BeamDeflectionLabApp (line 48)
            startApp(obj)

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);

I have tried everything under the sun to try and stop this from happening and I can only assume it is attempting to do something after the app has already been deleted.

I did find this Question: https://uk.mathworks.com/matlabcentral/answers/451421-packaged-app-error-r2018b that included the same errors, but there wasn't really a solution provided. Is there anything I can add into my code to correct this or an easy fix? I can share the app code if that makes diagnosis easier?

Thanks!