Currently Page-Previews has a hard dependency on TextExtracts and PageImages but those are required only when PopupsGateway is set to mwApiPlain.
We shouldn't depend on extensions that are not required to run. RestbaseHTML is a default gateway on production wikis.
Require TextExtracts and PageImages only when gateway is set to mwApiPlain.
Logic is inside PopupsContext::areDependenciesMet and will need tests.