Relation widget crashes if valueField doesn't return string or component.

[tech4him1]

- Do you want to request a feature or report a bug?
bug

- What is the current behavior?
If the valueField for a relation widget isn't a string (or other renderable object in React), the CMS will crash.
- If the current behavior is a bug, please provide the steps to reproduce.
Example:

    fields:
      - label: "Related Post Date"
        name: "post_date"
        widget: "relation"
        collection: "posts"
        searchFields: ["title", "date"]
        valueField: "date"

Try to type into the relation widget.
- What is the expected behavior?
The relation widget should still work with fields that don't return strings or React components.

- Please mention your CMS, node.js, and operating system version.

1.2.0-dev

- Please link or paste your config.yml below if applicable.

[tech4him1]

This could potentially be "fixed" by converting the values to strings before including them in the relation widget. It would probably need done at: https://github.com/netlify/netlify-cms/blob/952a6d509f4e681d4757bef51d4b2011c72f77ad/src/components/EditorWidgets/Relation/RelationControl.js#L90 and https://github.com/netlify/netlify-cms/blob/952a6d509f4e681d4757bef51d4b2011c72f77ad/src/components/EditorWidgets/Relation/RelationControl.js#L106
I don't see any problem with just converting the value to a string when suggesting options (first code), but I don't like doing that for the actual saved value (second code). The saved value in the entry should be the actual value that's in the related post. We'll need to see if the second only affects the suggestion, or the actual saved value.

[tech4him1]

Ah, looks like the actual saved value is captured separately:
https://github.com/netlify/netlify-cms/blob/952a6d509f4e681d4757bef51d4b2011c72f77ad/src/components/EditorWidgets/Relation/RelationControl.js#L66 https://github.com/netlify/netlify-cms/blob/952a6d509f4e681d4757bef51d4b2011c72f77ad/src/components/EditorWidgets/Relation/RelationControl.js#L84

My simple fix proposed above should work, then.

[barthc]

Issue fixed in #1936