Container: only Control/Container can be added to form (BC break)

Author: dg

src/Forms/Container.php

@@ -290,6 +290,10 @@ public function addComponent(
 		?string $insertBefore = null,
 	): static
 	{
+		if (!$component instanceof Control && !$component instanceof self) {
+			throw new Nette\InvalidStateException("Component '$name' of type " . get_debug_type($component) . ' is not intended to be used in the form.');
+		}
+
 		parent::addComponent($component, $name, $insertBefore);
 		$this->currentGroup?->add($component);
 		return $this;