Spurious invisible breakpoints are causing errors

[martin-henz]

The only ways to recover at the moment are:

• try to click on the gutter at the offending line, or if that fails
• visit Source Academy in incognito mode

Here is a recording that shows that the breakpoint remains active when the line is deleted.

Screen.Recording.2023-09-02.at.4.14.25.PM.mov

(The breakpoint disappears when the line is reentered.)