\Drupal\workflows\Form\WorkflowTransitionAddForm::form sets a default value for radios as an empty array.

@Sam152 yeah that should do it.

Dug around a bit but doesn't seem to be test coverage for this field having nothing selected by default.

We should probably add a line to \Drupal\Tests\workflows\Functional\WorkflowUiTest

    $this->clickLink('Add a new transition');
    $this->submitForm(['id' => 'publish', 'label' => 'Publish', 'from[draft]' => 'draft', 'to' => 'published'], 'Save');
    $this->assertSession()->pageTextContains('Created Publish transition.');
    $workflow = $workflow_storage->loadUnchanged('test');
    $this->assertTrue($workflow->getTypePlugin()->getState('draft')->canTransitionTo('published'), 'Can transition from draft to published');

    $this->clickLink('Add a new transition');
    $this->submitForm(['id' => 'create_new_draft', 'label' => 'Create new draft', 'from[draft]' => 'draft', 'to' => 'draft'], 'Save');
    $this->assertSession()->pageTextContains('Created Create new draft transition.');
    $workflow = $workflow_storage->loadUnchanged('test');
    $this->assertTrue($workflow->getTypePlugin()->getState('draft')->canTransitionTo('draft'), 'Can transition from draft to draft');

This seems to be all the coverage the form has at the moment, or is there some more somewhere else?

+++ b/core/modules/workflows/tests/src/Functional/WorkflowUiTest.php
@@ -137,7 +137,7 @@ public function testWorkflowCreation() {
+    $this->assertSession()->checkboxNotChecked('to');

The problem with checkboxNotChecked (aside from the confusing use on a radio button) only checks the first element with the given name (it uses find(), not findAll()). Just as a quick test, if you change the default for 'to' to 'draft', this test still passes ('published' is the first radio button for 'to').

I would go for something like:

    $this->assertCount(2, $this->cssSelect('input[name="to"][type="radio"]'));
    $this->assertCount(0, $this->cssSelect('input[name="to"][checked="checked"][type="radio"]'));

Looks great now. Thanks!