If you have access to the IDE in cloud you can restart or terminate the session through the session menu.
You also have terminal access to the container through the IDE and can delete or move your .rstudio files manually (if that's the problem).
The project is coming up in our logs as hitting memory limit (1GB) on the container 15 times in the last 2 days. So I would guess that generating the histogram and displaying it is consuming too much memory - which then generates the error message when the rsession crashes.
Sean