I have to apologise - I got the same error! Was caused by a recent change: https://github.com/mupen64plus/mupen64plus-ui-console/commit/e147b94f3c82866a42e3f72820e73c441158e7d5#comment

it has been reverted following discussion here: https://github.com/mupen64plus/mupen64plus-ui-console/pull/61

Please try again and it should work.

Try changing the video driver in retroarch to vulkan or glcore (working for me). The gl implementation is currently busted but you can try the workaround: https://docs.libretro.com/library/dolphin/

There is also currently a bug with this core and the GL driver that can be worked around by going to settings > user interface > show advanced settings ON and then going to settings > core > allow cores to change the video driver OFF.
The Dolphin core will now work without issues.

I got this resolved on my system (Ubuntu 18.04, Intel i5-3470 iGPU) by installing and using Vulkan rendering. I did not have to download the Sys folder from the Dolphin source repo.

I must note that Vulkan is not supported on any CPU earlier than Ivy Bridge, so this may not be a viable solution for some people. Mine is the earliest possible generation chip this even works on, so I guess I got lucky.

Install Vulkan support and testing application

sudo apt install -y mesa-vulkan-drivers vulkan-utils

Test Vulkan (successful example):

vulkan-smoketest

Modify the GC and Wii cores' config files to use Vulkan renderer:

sed -i '$ i\video_driver = "vulkan"' /opt/retropie/configs/wii/retroarch.cfg sed -i '$ i\video_driver = "vulkan"' /opt/retropie/configs/gc/retroarch.cfg

After doing this, GC games worked fine. I'll come back and modify this post if necessary whenever I get around to adding Wii to my system.

Sorry to be of bother have now sorted it myself, according to the libretro docs you have to download a sys file to your bios folder. Can now say I have every systems working. Please keep up the excellent work with retropie

Thanks mitu! It works!!!!!

@mitu Thank you so much for your help. I was able to get it to work by updating gcc. It took me a bit but I was able to figure it out. I'm a linux noob but prefer it and retropie over anything windows has to offer. Thanks again for your patience and helping me through this!

@mitu sorry about the late response. So excited as it's building right now so thank you for the update and @BuZz for the fix!