I can reproduce this bug every time I restart my computer. The first click of the MintMenu always brings up this type of empty window.
One thing I noticed as I was taking this screenshot is that when I left my mouse on the empty window, the tooltip for 'Lock Screen' came up ("Requires password to unlock"). I tried to bring up other tooltips but was unsuccessful.
I can reproduce this bug every time I restart my computer. The first click of the MintMenu always brings up this type of empty window.
One thing I noticed as I was taking this screenshot is that when I left my mouse on the empty window, the tooltip for 'Lock Screen' came up ("Requires password to unlock"). I tried to bring up other tooltips but was unsuccessful.