ResourceEditor: Fix crash

Amends 9e9bbf215c.

Change-Id: I4856dc001b5e0f52f1c4f54d59fd72bd3369f553
Reviewed-by: hjk <hjk@qt.io>

src/plugins/resourceeditor/qrceditor/resourcefile.cpp

@@ -224,7 +224,8 @@ bool ResourceFile::save()
     }
 
     const Result<> res = m_textFileFormat.writeFile(m_filePath, contents());
-    m_error_message = res.error();
+    if (!res)
+        m_error_message = res.error();
     return res.has_value();
 }