Issue Summary

Redash hangs when I try to change a dashboard widget from using a widget-level parameter to using a dashboard level parameter. The javascript console provides the following error:

react-dom.production.min.js:17 Uncaught TypeError: Cannot read property 'value' of undefined
    at EditParameterMappingsDialog.jsx:23
    at Qt (lodash.js:639)
    at Ka (lodash.js:9553)
    at f (EditParameterMappingsDialog.jsx:15)
    at t.value (EditParameterMappingsDialog.jsx:65)
    at onOk (EditParameterMappingsDialog.jsx:96)
    at e.handleOk (Modal.js:96)
    at n.handleClick (button.js:126)
    at Object.<anonymous> (react-dom.production.min.js:15)
    at Object.invokeGuardedCallback (react-dom.production.min.js:16)

Here's a gif that demonstrates the bug:

Steps to Reproduce

1. Create a dashboard
2. Add a widget that uses a parameter. Use a widget-level parameter to begin with
3. Save the dashboard and then change the parameter to dashboard-level
4. Click save

Technical details:

• Redash Version: 7.0.0
• Browser/OS: Safari, Chrome, Firefox on MacOS Mojave
• How did you install Redash: SaaS