Merge branch 'develop' of https://github.com/getgrav/grav into 1.6

# Conflicts:
#	CHANGELOG.md

system/src/Grav/Framework/Session/Session.php

@@ -225,8 +225,10 @@ class Session implements SessionInterface
             $params['httponly']
         );
 
-        session_unset();
-        session_destroy();
+        if ($this->isSessionStarted()) {
+            session_unset();
+            session_destroy();
+        }
 
         $this->started = false;