Uncaught TypeError: Cannot read property 'line' of undefined

Getting this error trying to use this linter:

```
Uncaught TypeError: Cannot read property 'line' of undefined
at groupByLine (lint.js?ver=10.5.0:107)
at updateLinting (lint.js?ver=10.5.0:167)
at lint.js?ver=10.5.0:159
at runInOp (codemirror.js?ver=10.5.0:3921)
at CodeMirror.operation (codemirror.js?ver=10.5.0:8603)
at startLinting (lint.js?ver=10.5.0:159)
at lint.js?ver=10.5.0:206
```

Also, is this rep still being maintained?

Start with lint.js at groupByLine (line 107), then follow its callers in updateLinting and startLinting. Reproduce the linter use described in the report and inspect the undefined value behind the TypeError. Done means the reported use no longer raises this uncaught error and the existing CodeMirror linting flow still works.

Written by the indexing model from the issue text.