Mine has an X (exit icon) across the top, weird, it's like there is no border/decoration on that window.
If you right click on your bottom panel and choose edit panel you may be able to toggle Activities window's visibility on the small, screen edge menu with the "Activities" option.
Another option is to switch themes and choose to accept the settings of the new theme.
Cheers!
Edit:
The 'pine+tab' keystroke launches that window and I have to click on the window and press the 'esc' key to close it.
If you right click on your bottom panel and choose edit panel you may be able to toggle Activities window's visibility on the small, screen edge menu with the "Activities" option.
Another option is to switch themes and choose to accept the settings of the new theme.
Cheers!
Edit:
The 'pine+tab' keystroke launches that window and I have to click on the window and press the 'esc' key to close it.