Clean Class Browser after it was closed

[MariusBrill]

Is your feature request related to a problem? Please describe. Currently, whenever the class browser is closed and reopened, the last data fetched remains in the class browser. Hence, the window is not empty when reopened. However, as a user i would assume that the contents of the class browser is empty when reopening it.

Describe the solution you'd like When closing the class browser and opening it again, the class browser should be empty.