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If you are getting the error message "You used a NULL action in a workflow, use dummy node instead" in Odoo, it is likely that there is an issue with the workflow configuration. This error can occur when a workflow is trying to execute an action or activity that is not defined or does not exist.

To resolve this issue, you will need to check the workflow configuration and make sure that all of the actions and activities are defined and correct. To do this, go to the "Settings" menu and select the "Workflow" option. Then, click on the workflow that is giving the error, and go to the "Diagram" tab in the workflow form. In the diagram, check each of the nodes and transitions, and make sure that they are connected to valid actions and activities.

If you find any nodes or transitions that are not connected to valid actions or activities, you can remove them from the workflow by clicking on the node or transition and then clicking on the "Delete" button. You can then add new nodes and transitions that are connected to valid actions and activities, and reconfigure the workflow to make sure that it is correct and complete.

Once you have checked and reconfigured the workflow, you should be able to save the changes and avoid the error message. If the issue persists, you may need to check the Odoo logs for more information about the error, and contact the Odoo support team for assistance.