This issue is due to a change in the data source of your form (i.e. if you rename the SharePoint list, it will cause this problem).
Here are some steps you can take to resolve this issue:
Recreate the Form: One way to fix the issue is by deleting the custom form and recreating it.
Export and Modify the App: You can export your Power App, unzip the export, and then locate the .msapp file (which is the actual Power App). Make a backup copy and change the .msapp file extension to .zip. Extract the zip and you will see a few folders with a bunch of JSON files in it. One of those files has the logic that defines the SharePointIntegration object. If you have ever renamed your data source on the form, you will find that when you export the Power App, one of the references does not get updated. You can manually update this reference, re-zip the subfolder and rename the zip back to .msapp, re-zip the entire Power App zip package and import it back.
Use Browser Console: You can also try to fix the issue using the browser development console (F12). Go to the form where you got the SharePointIntegration.SelectedHidden error in the Power Apps editor and type in the instructions in the console. Change all the references to data sources in your app that use the .ID column to .'Id (ID)'.
If you don't want to try any of the steps above, you can try to edit the SharePointForm1.Item property as follows:
If(IsBlank(SharePointIntegration.SelectedListItemID),First([@'TheNameOfYourListGoesHere']),LookUp([@'TheNameOfYourListGoesHere'],ID=SharePointIntegration.SelectedListItemID))