[BUGFIX] Fix wrong condition for identifying allowed edit mode

Classes/Controller/AbstractBackendController.php

@@ -54,7 +54,7 @@ abstract class AbstractBackendController extends AbstractController {
 		$editingMode = $this->session->getDataByKey('editingMode');
 		$availableEditingModes = $this->configurationService->getAvailableEditingModes();
 		if ($this->backendUser->isAdmin()) {
-			if (array_key_exists($editingMode, $availableEditingModes)) {
+			if (!array_key_exists($editingMode, $availableEditingModes)) {
 				$firstAvailableEditMode = key($availableEditingModes);
 				$this->session->setDataByKey('editingMode', $firstAvailableEditMode);
 			}
@@ -62,7 +62,7 @@ abstract class AbstractBackendController extends AbstractController {
 		} else {
 			$canChangeEditingModes = count($availableEditingModes) > 0
 			                         && $this->backendUser->user['lfeditor_change_editing_modes'] !== 0;
-			if (!$canChangeEditingModes || empty($editingMode)) {
+			if (!$canChangeEditingModes || !array_key_exists($editingMode, $availableEditingModes)) {
 				end($availableEditingModes);
 				$lastAvailableEditMode = key($availableEditingModes);
 				$this->session->setDataByKey('editingMode', $lastAvailableEditMode);