Do you have the WA Site Tools panel docked or does it float on the left side? If it is not docked then you might want to try moving it over to the sidebar where the files and application panel are.
If you get the same result it might be a problem with the cache file. If this is the case here is a link to the steps you can take for that:
If the issue still persists there could be a corruption of your configuration folder, here is a link with steps to deal with that:
Let us know if you continue to have this problem after trying these things.