Are you really importing the Aperture Library or are you opening the Aperture Library in Photos for Mac to migrate it to Photos? Importing will not work, because Photos cannot merge libraries into existing Photos Libraries, but Photos should be able to open the library and to convert it, as described here:
Migrate your Aperture libraries to Photos or Adobe Lightroom Classic - Apple Support
I also had repeated crashes when I migrated older Aperture Libraries to Photos.
It helped to upgrade the older libraries by opening them in Aperture 3.6. Older Aperture Libraries (Aperture 3.3 or older) were problematic. If the libraries have already been upgraded to Aperture 3.6 and Photos is still crashing on them, check, if Photos already created a new Photos Library and if Photos can open this library. If you can open the library inPhotos, create a smart album (File > New Smart Album) with the rule "Photo is unable to upload to iCloud Photos". This smart album will show items, that are in an incompatible format - unsupported video codecs, unsupported color profiles, etc, PDF files or Audio Files.. Delete these items from the library and import converted items instead.
Frequently it helps to export the problematic items from Photos and then to reimport them. Photos can sometimes convert the items on import to a more compatible format. But not during the migration from Aperture.