TYPO3 Core

Test is:

formDefinition:

type: Form
identifier: test
label: test
prototypeName: standard
renderables:
  -
    type: Page
    identifier: page-1
    label: Step
    renderables:
      -
        type: Number
        identifier: number-1
        label: Number

defaultValue is not set but should be validated within the form editor.

No fake an incomplete setup.
Goto EXT:form/Configuration/Yaml/FormEditorSetup.yaml and search for

          TextMixin:
            formEditor:
              predefinedDefaults:
                defaultValue: ''

remove the line with 'defaultValue'

          TextMixin:
            formEditor:
              predefinedDefaults:
#                defaultValue: ''

Clear caches, go to the form editor and open this form.
Without the patch, the form editor breaks and show the spinner endless.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56326

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56326

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56326

Patch set 1 for branch TYPO3_8-7 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56716