TextEditor: Fix crash

Introduced in dcba4a27

Occurs when invoking InsertVirtualMethods to implementation file, which
is not opened.

Change-Id: Icc84d63ddf28312eb77f2666166f609085d81d63
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/texteditor/refactoringchanges.cpp

@@ -393,7 +393,7 @@ void RefactoringFile::indentOrReindent(void (RefactoringChangesData::*mf)(const
         QTextCursor selection(p.first.document());
         selection.setPosition(p.first.position());
         selection.setPosition(p.second.position(), QTextCursor::KeepAnchor);
-        ((*m_data).*(mf))(selection, m_fileName, m_editor->baseTextDocument());
+        ((*m_data).*(mf))(selection, m_fileName, m_editor ? m_editor->baseTextDocument() : 0);
     }
 }