How to reproduce:
1. Create a new project in your prefered Plastic workspace
2. Open it and connect it to Plastic
3. Create a new script and submit it
4. Check out the script and in its Inspector click "Lock"
5. Observe the error and the VCS connection

Expected result: file locking functionality works without errors or disconnects
Actual result: a "Version Control: PlasticSCM : Command not implemented: [lock]" error is thrown and version control is disconnected

Reproduced in: 2019.3.0a11, 2019.3.0a4
Not reproducible with: 2019.3.0a3, 2019.2.1f1, 2019.1.14f1, 2018.4.6f1, 2017.4.31f1 (No "Lock" button in the file's Inspector)

Note: Locking files using Plastic works without issues but unlocking them through the Editor still throws an error and disconnects VCS