Editor: Fix crash on switching sessions.

Task-number: QTCREATORBUG-9220

Change-Id: I0671eaa1e9785ee0327bc8cbd4519c5d696ea0ca
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/editormanager/editormanager.cpp

@@ -2247,7 +2247,8 @@ void EditorManager::removeAllSplits()
     d->m_splitter->unsplitAll();
     if (!editor)
         editor = pickUnusedEditor();
-    activateEditor(editor);
+    if (editor)
+        activateEditor(editor);
 }
 
 void EditorManager::gotoOtherSplit()