Fixed [4.0 b3] Error loading non-existent DB

[Wanderer]

1. Load or create a DB in RC.
2. Close RC without closing the DB first.
3. Remove the DB file.
4. Load RC again.
5. Click on the DB tab.

Result: "Error: file not found" (logical message) but right after pressing OK on that, "Object reference not set to an instance of an object".

RC stays alive so it seems the error is handler properly but IMO instead of letting it show the "Object reference" error, it would be better to ask the user if he wishes the tab to be removed from the tabs list.