Add useful exception when hook class is invalid/missing (see #1949)

Description
-----------

If an invalid class is imported, we'd always get `Warning: in_array() expects parameter 2 to be array, null given` because `get_class_methods()` returns null instead of an array.

Commits
-------

1cf46e6 Add useful exception when hook class is invalid/missing
d5bc83c CS

core-bundle/src/Resources/contao/library/Contao/System.php

@@ -161,6 +161,10 @@ protected function import($strClass, $strKey=null, $blnForce=false)
 			{
 				$this->arrObjects[$strKey] = $container->get($strClass);
 			}
+			elseif (!class_exists($strClass))
+			{
+				throw new \RuntimeException('System::import() failed because class "' . $strClass . '" is not a valid class name or does not exist.');
+			}
 			elseif (\in_array('getInstance', get_class_methods($strClass)))
 			{
 				$this->arrObjects[$strKey] = \call_user_func(array($strClass, 'getInstance'));
@@ -202,6 +206,10 @@ public static function importStatic($strClass, $strKey=null, $blnForce=false)
 			{
 				static::$arrStaticObjects[$strKey] = $container->get($strClass);
 			}
+			elseif (!class_exists($strClass))
+			{
+				throw new \RuntimeException('System::importStatic() failed because class "' . $strClass . '" is not a valid class name or does not exist.');
+			}
 			elseif (\in_array('getInstance', get_class_methods($strClass)))
 			{
 				static::$arrStaticObjects[$strKey] = \call_user_func(array($strClass, 'getInstance'));