fastboot boot image-new.img
You will boot with root but modules disabled. After you remove the offending module you will need to go to /cache folder and delete the .disable_magisk file before your modules will work.
Image uploaded 12-5-19
This one should remove magisk and all modules, then reboot and magisk should reinstall itself (ask to install necessary binaries). This is what osm0sis uses to recover from failed flashes. See this post: