the Joiner node, among others, on my Xubuntu 18.04 system has the annoying tendency of showing a mostly empty window when the input table no longer contains the column names that the Joiner was configured with. Resizing the window makes all the UI elements appear again.
Edit: this also happens on the regular Ubuntu 18.04.
Sometimes this happens on Windows 10 too if you copy another Joiner node which has settings / columns that are not present at the node’s new location. That the view would freeze. It might be that there is some ‘refresh’ missing.