This is not a tutorial, it’s more of a note so I’m going to remember how to do something the next time I stumble again on the problem.

Disclaimer: I’m not responsible if any of the following methods is going to delete any of your files. You really should make a backup before attempting it. Mostly your Xcode preferences should be kept, but it is not a guarantee.

Best solution

[This method only works if you have a paid developer account.]

• Go to apple download page
• Click on more at the bottom
• Download the latest version of Xcode
• Make sure to delete the MacAppStore version of Xcode before updating it.

If you really want to use the MacAppStore, first ask yourself why. After that try these methods:

Method 1

[This method didn’t work for more, but worked for others]

• Turn on debug mode for Appstore. Open the Terminal and write defaults write com.apple.appstore ShowDebugMenu -bool true
• Relaunch Appstore
• From the Menu select: Debug → Reset Application.

Method 2

[This method worked for me without deleting any preferences or add-ons]

• Delete code
• Quit the MacAppStore
• Restart your Mac
• Re-download Xcode ex-novo from the MacAppStore