There are a few reasons why Sketch might crash after you've installed Craft:
You might be using an older version of Sketch
Because Sketch is frequently updated with new functionality, older versions of Sketch can sometimes have issues with some plugins. If you're having trouble with Sketch crashing, we recommend that you update to the latest stable version of Sketch for the best experience. Please see the Sketch version history if not sure what the latest version is.
You might have hidden the Inspector
With the initial launch of Craft, we discovered an issue that can cause Sketch to crash on startup if the Inspector was hidden. This has been fixed in the latest version of Craft but if you're using an older version of Craft and you'd hidden the Inspector, then you can fix this by doing the following:
Open the Activity Monitor app in OS X
In the upper right corner, search for "Panels" and click on the app that's listed. Panels is the companion app for Craft that makes all the magic happen.
Click on the stop sign icon in the upper left corner
Click Quit to close Panels
Reopen the Sketch app
Click on View > Show Inspector and make sure it's checked
Relaunch the Panels app from your Applications folder
After that, Sketch should no longer crash if the issue was caused by the hidden Inspector.
Though these steps will temporarily resolve the issue on older versions of Craft, you should update Craft by downloading the newest version.