If you click too fast in the Java Realms menu's, the game promptly crashes. It's very easy to reproduce: for example, reset the world, and just before the world slots show up again, click where they would be after loading. Also see https://gfycat.com/ellipticaldefiniteblueandgoldmackaw (black screen means that the game crashed). Crash report is attached.
It's annoying for those not patient enough to not click prematurely. And while I don't think that you can have no loading at all, I suggest that the click just does nothing instead.
There was a fix in Minecraft 1.15.1 that supposedly solved this issue. Is this still a problem for you?