Could you please provide more information to assist you further:
When does Lightroom closes? while importing or while working in develop module?
Which version of Lightroom are you using?
Does it crashes while working?
When I export files...
When I interact with Photoshop, Nik Collection Software, Perfect Effects...
It's was perfect until I upgrade my Mac to OS El Capitan, and upgrade the new version of Lightroom two days before...
Would suggest you to go to Lightroom Preferences> General and disable the option "Show Add Photos Screen"
Relaunch Lightroom and check if the issue persists.
Thanks for the support Chetna...
I re-install Lightroom, Perfect Photo Suite and Nik Collection... then I disable the option "Show Add Photos Screen"... now everything is perfect again!