This issue occurs when you do backup restore of the site/site collection. Due to this you can not see the page in design mode due to some reference changes and will not be able to add any new data sources on the page.
To resolve this issue follow below steps.
1. Go to Insert - New Item Form
2. Click on Custom List Form
3. Choose your custom list.
4. Choose form type (new/edit/display).
5. Select the "DataSources" section of the newly created dataformwebpart.
6. Now replace the existing "Datasources" section with selected one.
7. Remove the old DataFormweb part.
Just after saving the form mentioned error should go and you should be able to see all the controls with existing binding.
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