Been building loads of custom races and copy/changes of the default ones too. No issues here on my end with crashes. Only thing i can suggest is making sure that you have your custom images in the correct size/format. Can also check to make sure that the 'my games' folder is accessible by the game in permissions.

Hello,

We will have a fix for this in our next patch.

The opt-in for the patch is up now if you want to check it out.  

To enable the patch, please do the following:

• Select Galactic Civilization III in your Steam Library list
• Right-click and select Properties
• Select the BETAS tab
• From the dropdown, select opt-in and click Close (if you don't see this option, restart Steam)

Details are here:

https://forums.galciv3.com/473036/page/1/#3605937