Prevent dereferencing an invalid file pointer in the plain text editor.

src/plugins/texteditor/plaintexteditor.cpp

@@ -164,6 +164,7 @@ void PlainTextEditor::setTabSettings(const TextEditor::TabSettings &ts)
 
 void PlainTextEditor::configure()
 {
+    if (file())
         configure(Core::ICore::instance()->mimeDatabase()->findByFile(file()->fileName()));
 }