Studio auto generated update form with relationship field throws error

### Before opening, please confirm:

- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-adminui/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-adminui/blob/main/CONTRIBUTING.md#bug-reports).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.
- [X] I have removed any sensitive information from my code snippets and submission.

### App Id

d1d64siqsqavbr

### Region

us-east-1

### Environment name

dev

### Figma File Version (if applicable)

_No response_

### Amplify CLI Version

_No response_

### If applicable, what version of Node.js are you using?

_No response_

### What operating system are you using?

_No response_

### Browser type?

_No response_

### Describe the bug

When using the auto generated update form with a relationship file throws `The source object is not a valid model` error when selecting a relationship field `Add item` but not adding the item using `add` then submitting the form throws the unhelpful error.

but works normal when not adding the relationship

### Expected behavior

throw meaningful error or cause

### Reproduction steps

1. use a update form
2. in the relationship field try adding a item but dont add the relationship using `add` button
3. click submit
4. observe error

### Project Identifier

_No response_

### Additional information

_No response_

Start by reproducing the issue with an auto-generated update form and a relationship field: select Add item, do not confirm it with Add, then submit. Trace the relationship-field submission path to identify why the source object is rejected; done means the case produces a meaningful error or cause instead of the reported unhelpful error.

Written by the indexing model from the issue text.