ViewsEntitySchemaSubscriber may fail when a view has a broken handler

#3 The patch was not applied for to 8.7.x branch

Patch #4 worked for me for updating from 8.6.(don't remember) to 8.7.3.

+++ b/core/modules/views/src/EventSubscriber/ViewsEntitySchemaSubscriber.php
@@ -276,7 +276,9 @@ protected function processHandlers(array $all_views, callable $process) {
+            if (is_array($handler_config)) {

Why would this not be an array? That sounds like broken config. So are we sure this is not just hiding a valid failing scenario?

I think we should reopen this: we cannot risk to break the update process because a view cannot be resaved, we already committed similar fixes in the past weeks.

This tries a slightly different approach, since each callback might decide to handle a NULL value differently.

A conceptually similar fix was applied in #3052492: ViewsEntitySchemaSubscriber should not make an entity update fail if a view cannot be resaved.

It looks like we could use the view from core/modules/views/tests/modules/views_test_config/test_views/views.view.test_view_broken.yml for the test coverage here.

This should provide test coverage, the test-only patch is the interdiff.

I could not rely on the broken view suggested by @amateescu because that does not return a NULL handler and it seems it is actually impossible to save a test view with a NULL handler value, so I had to work around that.

The test-only patch was broken, reposting it.

Fixed a couple of PHP docs issues.

... and the interdiff.

Looks great to me! Thanks for sticking with this, @plach :)

It looks like this could fix a lot of the problems for people updating to Drupal 8.7.x (based on #7), so tagging accordingly.

Committed/pushed to 8.8.x and cherry-picked to 8.7.x, thanks!