Fatal error when creating a deliverer with existing name

Added method and exists check

Added test method and moved checks and function in DelivererConfigurationForm.

+++ b/src/Form/DelivererConfigurationForm.php
@@ -38,6 +52,18 @@ class DelivererConfigurationForm extends PluginConfigurationForm {
+    if ($this->delivererExists($form_state->getValue('label'))) {
+      $form_state->setErrorByName('label', $this->t('A plugin with label @label already exists.', ['@label' => $form_state->getValue('label')]));
+    }

Not sure if we should disable creating deliverers with the same name. It should not cause any problems since it's not a primary key.

Also, test-only patch helps to reveal a bug/fatal error in such cases.

1.Removed delivererExists function, only checking the id.

I'm wondering when this stopped working... There is a validation added on #machine_name element in PluginConfigurationForm that should prevent those cases. Could be a core bug or so.

+++ b/src/Form/DelivererConfigurationForm.php
@@ -38,6 +38,15 @@ class DelivererConfigurationForm extends PluginConfigurationForm {
+      $form_state->setErrorByName('id', $this->t('A plugin with @id Machine-readable name already exists.', ['@id' => $form_state->getValue('id')]));

Maybe we can remove "Machine-readable name" from the sentence.

1.Removed 'machine-readable name' from message.

1. Forgot to update the test. (correct patch here)

Removed unrelated change.

Adding test-only patch and the patch with the validation method.

Is it much better now ;) only, be careful with uploading patch. Check patch before uploading.

We need some more investigation (examples) to confirm it's a core bug, but we're getting that way, since this issue occurs in Collect as well #2826323: Fatal error when creating collect model with existing name.

There is a bug with this patch in the validate function. When editing a deliverer and trying to save it, there is a message that reports there is a deliverer with the same machine name.
#2557299: Any AJAX call disregards machine name verification when AJAX is used and leads to a fatal error This issue covers the fatal error, and I guess when it is fixed, there will be no fatal error reported anymore.
I suggest that we revert the patch and wait for the core issue to be fixed instead.
Here is a patch removing the code from this issue.