Somebody messaged me with an error they were experiencing. Thought I'd post it here for everybody to benefit from. Here was the error they got in their Umbraco log:

2017-01-04 14:40:12,622 [P3932/D12/T185] ERROR formulate.app.Controllers.FieldsController - An error occurred while attempting to get the Field Categories for a field.
System.ArgumentNullException: Value cannot be null.
Parameter name: source
at System.Linq.Enumerable.Cast[TResult](IEnumerable source)
at formulate.app.Managers.DefaultConfigurationManager.get_FieldCategories() in C:\r\formulate\src\formulate.app\Managers\DefaultConfigurationManager.cs:line 148
at formulate.app.Controllers.FieldsController.GetFieldCategories() in C:\r\formulate\src\formulate.app\Controllers\FieldsController.cs:line 199
2017-01-04 14:40:12,627 [P3932/D12/T36] ERROR formulate.app.Controllers.FormsController - An error occurred while attempting to get the form info for a Formulate form.
System.ArgumentNullException: Value cannot be null.
Parameter name: source
at System.Linq.Enumerable.Cast[TResult](IEnumerable source)
at formulate.app.Managers.DefaultConfigurationManager.get_FieldCategories() in C:\r\formulate\src\formulate.app\Managers\DefaultConfigurationManager.cs:line 148
at formulate.app.Controllers.FormsController.GetFormInfo(GetFormInfoRequest request) in C:\r\formulate\src\formulate.app\Controllers\FormsController.cs:line 122

I believe this can happen if an upgrade goes wrong. To fix, uninstall and reinstall Formulate (you may have to also delete all mentions of Formulate from the web.config when you uninstall).

If that still doesn't work, add the fieldCategories.config folder to your site under config/formulate: https://github.com/rhythmagency/formulate/blob/cdb7a08717b0874666acd11503b20655759d73b4/src/Website/Config/Formulate/fieldCategories.config

You may also have to update the web.config. Search for "fieldCategories" in this file to see the necessary changes: https://github.com/rhythmagency/formulate/blob/cdb7a08717b0874666acd11503b20655759d73b4/src/formulate.app/App_Plugins/formulate/Transforms/web.config.install.xdt.txt