It appears as though the error is misleading. If you keep getting "Error creating Heroku Tool Window", here is the way I fixed it.
Open up the settings in your Idea IDE.
Under "IDE Settings", click on the words Heroku and you should see the correct error "Missing API-Key, Please retrieve the Heorku-API Key from your Heroku Account Page"
Login to your account at www.heroku.com
Click on the icon at the top right
Click on "Account" in the drop down
Click the "Show API Key" button in the API Key section
Enter the password
Click the "Show API Key" button
Copy the string into your Idea IDE under the "Heroku Credentials" Section
Click "Check API-Key"
Click "OK" after you see "Heorku Login, Heroku Login successful! Authorized:"