Implement support to invalidate custom property pane controls
At the moment custom property pane controls cannot invalidate the whole property pane. This means if you enter an incorrect value for your custom input, the property pane will still apply it (or the apply button will be enabled).
It would be great to have a way to make it possible to invalidate the property pane like the OOTB property pane controls. This would have a huge benefit for all the controls of the @pnp/spfx-property-controls (https://github.com/SharePoint/sp-dev-fx-property-controls) project which is used in a lot of projects.
More details for this can be found in this issue on Github: https://github.com/SharePoint/sp-dev-docs/issues/3440