Photos keeps crashing on iMac running High Sierrra

Is your Photos Library on a compatible drive as described here: Move your Photos library to save space on your Mac - Apple Support

Do you have enough free storage on your system drive and the drive with the library?

Did you recently import new media files into the library - new photos or videos?

Try to narrow down the problem with a few tests:

• Try to find out, if the crashes only happen for your current Photos Library.
  You do that by creating a new library and importing a few photos for testing.
  See: Create additional photo libraries in Photos on Mac - Apple Support
• If the crashes happen also on a new library, check if the problem is system wide or only happens in your main account. You can do that by testing in a different user account. See this document: Isolating an issue by using another user account
• If the problem is system wide, test if you installed incompatible system extensions by booting into Save Mode. This will clear caches and temporarily disable this party system extensions, so you will be testing with a clean system: Mac OS X: Starting up in Safe Mode