only in patch2:
unchanged:
--- a/core/modules/entity/src/Form/EntityDisplayModeAddForm.php
+++ b/core/modules/entity/src/Form/EntityDisplayModeAddForm.php
@@ -52,7 +52,7 @@ protected function prepareEntity() {
       throw new NotFoundHttpException();
     }
 
-    $this->entity->targetEntityType = $this->targetEntityTypeId;
+    $this->entity->setTargetEntityTypeId($this->targetEntityTypeId);
   }
 
 }
only in patch2:
unchanged:
--- a/core/modules/entity/src/Form/EntityFormModeAddForm.php
+++ b/core/modules/entity/src/Form/EntityFormModeAddForm.php
@@ -23,7 +23,7 @@ protected function prepareEntity() {
       throw new NotFoundHttpException();
     }
 
-    $this->entity->targetEntityType = $this->targetEntityTypeId;
+    $this->entity->setTargetEntityTypeId($this->targetEntityTypeId);
   }
 
 }