The uialert window cannot be programmatically closed, at least not in r2020a. You can, however, make the uialert non-modal which will allow you to interact with the base figure (app) while the uialert still exists.
uialert(uifig, 'message', 'title', 'Modal', false)
Alternatively, you could use a msgbox which outputs a handle that you can use to delete at any time but this will not be automatically centered on the app like the uialert panel does.
