objectManager: Show a note if a parent entry is missing

include/objectManager.php

@@ -1217,6 +1217,11 @@ class ObjectManager
             if($sLinkText == 'default')
             {
                 $oParent = $oObject->objectGetParent();
+                if(!$oParent)
+                {
+                    echo html_note('Failed to find parent entry', 'No parent entry could be found, even though it is supposed to exist.');
+                } else
+                {
                     $oParentOM = new objectManager(get_class($oParent), '', $oParent->objectGetId());
                     $sClassName = $oParentOM->getOptionalSetting('objectGetClassDisplayName', false);
                     if($sClassName)
@@ -1224,6 +1229,7 @@ class ObjectManager
                     else
                         $sLinkText = 'Move to another parent entry';
                 }
+            }
             echo "<a href=\"".$this->makeUrl("showChangeParent", $this->iId,
                  "Move to another parent entry")."\">$sLinkText</a>\n";
         }