Kibana version: master at f0f192c

Describe the bug:

When transitioning to Last value from an operation with a string field set, the editor crashes at validation time.

Steps to reproduce:

1. Open Lens editor and drop an field into the workspace
2. Click on the Vertical axis dimension "Count of Records"
3. Now click on "Unique count" and select a string field
4. Click on "Last value"
5. 💥

Expected behavior:

The field is marked as non valid and a warning is shown to the user about changing to a valid value.

Screenshots (if relevant):

Errors in browser console (if relevant):

Uncaught TypeError: Cannot read property 'sortField' of undefined
    at Object.getErrorMessage (last_value.tsx:126)
    at layer_helpers.ts:801
    at Array.forEach (<anonymous>)
    at getErrorMessages (layer_helpers.ts:790)
    at indexpattern.tsx:303
    at Array.map (<anonymous>)
    at Object.getErrorMessages (indexpattern.tsx:300)
    at validateDatasourceAndVisualization (state_helpers.ts:78)
    at workspace_panel.tsx:72
    at updateMemo (react-dom.development.js:16854)