Fix an assertReadAccessAllowed exception

The `la-clojure` plugin has been generating assertion Throwables in my [nightly IJ](https://github.com/JetBrains/intellij-community/commit/b913f1cd) builds for a few days now. The assertion text is extremely helpful, and it was a quick fix:

> Read access is allowed from event dispatch thread or inside read-action only
> (see com.intellij.openapi.application.Application.runReadAction())

It even appears from the unused imports(!) in the file that the `PsiManager` once upon a time did use `ApplicationManager`, so that was further confirmation that this is probably the correct fix.

The same number of tests are broken after my fix as were broken before it.