In case when spelling check is off (for example, Hunspell is set to off in
Options dialog), you cannot enable the "Check grammar" dialog box if it was
disabled. The spelling dialog closes instantly and there is no option in Options
dialog to enable it back.

The only workaround now is to enable hunspell, make a spelling mistake in a
language that has a spelling dictionary installed, and check the "Check grammar"
box again. There should be a standalone option as well in Options dialog
(Language settings > Spelling), as the workaround is not intuitive and there can
be usage scenarios when someone wants to disable spelling for a language (for
example, some languages could have no spelling dictionary but only a grammar
checker that includes some spelling functions).