What does the error message say? Word-for-word, do not paraphrase
After quitting LR (after a fe seconds) " Adobe Lightroom quit unexpectedly"
I had the same experience after upgrading from Lightroom 5. The problem persisted even after upgrading to Lightroom 6.8. As a start, I deleted two Lightroom preference files; "com.adobe.Lightroom6.plist", and "com.adobe.Lightroom6.LSSharedFileList.plist". After doing this I relaunched Lightroom. While opening, Lightroom displayed a dialog indicating that one of my plugins was now incompatible and needed to be updated. I clicked Okay and a dialog was displayed indicating that Lightroom would disable the plugin. I clicked Continue (or Okay I don't remember which) and Lightroom opened normally. I did some editing and then quit Lightroom. The error message did not reappear! In my case, the incompatible plugin was a Flickr upload plugin.
Preference files are located in library/preferences folder. If you are using macOS Sierra or the previous two OS versions, in-order to find the library folder, you will need to hold down the option key as you click on the Go menu in the Finder.