I doubt you're doing anything wrong. You might have an autosave problem where Xcode had trouble autosaving the project that keeps crashing. Go to the following location on your Mac:
/Users/YourUsername/Library/Autosave Information
Delete anything in there related to Xcode, such as files named Unsaved Xcode Document.xcworkspace. Restart Xcode and see if that fixes it.
Another thing to try is to select the project file from the file, right-click, and choose Show Package Contents. See if any of the files in the package were modified recently. Any recently modified file would be a candidate for causing the crash. You could move the file out of the file package and see if this stops the crash, but this could cause major problems with your project. Don't move the project.pbxproj file out of the file package. Make a duplicate of your project file before you start moving files out of the project file package.
The worst-case scenario is that your project file is corrupted. If you have a backup of your project, restore the backup. If you don't have a backup you'll have to create a new project and add the source code and xib files from the old project to the new project.