This error is most commonly seen when the workspace is open in a different location. In order to retain project coherency, you should find where SDK is open, and close it before trying the workaround below.
If youare certain that there are no open instances of the workspace open anywhere (i.e. improper shutdown of the tool), then you can delete this file: <workspace_dir>/.metadata/.LOCK
After this step, you should be able to open the workspace.