I did the same thing. Here's how I fixed it: I ran psar dumper through the recovery menu by putting it in the psp/game/recovery folder and running it as if it was the recovery program. I used it to dump the psar from the 3.10 update eboot, took the topmenu_plugin file from the firmware dump, and put it back where it belonged using the 3.10 OE recovery menu's ability to write flash0 via USB. It worked perfectly for me. The place in flash0 where you need to put the file will be the equivalent of where it was in the dump. If you need more info, let me know.

Also, I can email you the original file if you wish. Please let me know.