I have not used Osmo, but what you describe appears to be a bug or very bad
practice from the developers.
In C and C++ assertions are a feature intended to be used for internal
consistency checks (those that depend only in the program and not in its
environment). Sometimes they are used for external consistency checks, like
checking that a required file exists. This is incorrect. It is also very bad
practice because when the assertion fails it terminates the program suddenly
and gives no information for troubleshooting which is exactly what is
happening here.
If you changed something relevant, as a workaround try undoing it and try to
run Osmo again. You can also try compiling and installing the latest version
of gtkspell manually and instructing Osmo to use it. Maybe the bug has
already been fixed in the latest version; otherwise, report the bug to the
developers of Osmo.
