Infinite spinner loading when editing theme configuration

[Zyles]

Preconditions (*)

Magento 2.3.0

Steps to reproduce (*)

1. Go into Content -> Configuration
2. Click Edit on the active theme
3. Greeted by a forever spinner and unable to change the theme to another one or make any changes

Error in Chrome:

Uncaught ReferenceError: Base64 is not defined
at UiClass.processFile (file-uploader.js:173)
at Array.map ()
at UiClass.setInitialValue (file-uploader.js:78)
at UiClass.initialize (abstract.js:77)
at UiClass.initialize (wrapper.js:109)
at new UiClass (class.js:49)
at Object.initComponent (layout.js:137)
at fire (jquery.js:3232)
at Object.fireWith [as resolveWith] (jquery.js:3362)
at Object.deferred

Expected result (*)

Be able to edit theme configuration.

Actual result (*)

Spinning forever.

[magento-engcom-team]

Hi @Zyles. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento-engcom-team give me 2.3-develop instance - upcoming 2.3.x release

For more details, please, review the Magento Contributor Assistant documentation.

@Zyles do you confirm that you was able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

[magento-engcom-team]

Hi @engcom-backlog-nazar. Thank you for working on this issue.
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[ghost]

HI @ thank you for you report, this is duplicate to #19549