How to remove Error: 'Invalid api version:0.0' in Salesforce Developer Console?

This may be happen because you may have not mentioned the api version.

You may try doing this:

1. Copy your Class code to notepad
2. Switch to a new workspace (if you only have the default, create another one)
3. Switch Back to the workspace you were working on
4. Copy your notepad saved code to the class
5. Try to save your code again