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sObject type 'XXXX__c' is not supported.
Please uncheck the 'Use Tooling API' Checkbox in Query Editor whenever you see sObject type 'XXXX__c' is not supported error while running SOQL.
It is just after 'Execute' button. Please see the snapshot below:

It is just after 'Execute' button. Please see the snapshot below:

This is not question... it is just information in case if you see the same issue while running SOQL in Query Editor.