renderPageContent only works in production #487

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Bug report

api.renderer.renderPageContent only works in production mode.

Steps to reproduce

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Create a project with saber-plugin-search with the index option including content.

What is expected?

content should be set properly, as it is in production mode.

What is actually happening?

https://github.com/saberland/saber/blob/781995529eb66eb219af344e13feb039662aa1f5/packages/saber-plugin-search/lib/index.js#L47-L49

renderPageContent throws an error.

[error] TypeError: Cannot read property 'renderToString' of undefined
    at VueRenderer.renderPageContent (~/node_modules/saber/vue-renderer/lib/index.js:292:38)
    at Promise.all.map (~/node_modules/saber/packages/saber-plugin-search/lib/index.js:48:36)
    at Array.map (<anonymous>)
    at generateLocale (~/node_modules/saber/packages/saber-plugin-search/lib/index.js:31:31)
    at Promise.all.allLocalePaths.map.localePath (~/node_modules/saber/packages/saber-plugin-search/lib/index.js:67:40)
    at Array.map (<anonymous>)
    at generateDatabase (~/node_modules/saber/packages/saber-plugin-search/lib/index.js:67:22)
    at api.hooks.onCreatePages.tapPromise (~/node_modules/saber/packages/saber-plugin-search/lib/index.js:83:18)
    at tapInfo.fn.args (~/node_modules/saber/lib/index.js:88:20)
    at Promise (eval at create (~/node_modules/tapable/lib/HookCodeFactory.js:74:10), <anonymous>:46:17)
error Command failed with exit code 1.

Other relevant information

• Result of running saber -v: latest
• Browser version (optional):
• Is Saber a global or local install? local
• Which package manager did you use for the install? yarn
• Does this issue occur when all plugins are disabled?