Fixing missing instantiation error(s)

[olemartinorg]

Description

This fixes the missing instantiation error that didn't appear when creating a new instance from the instance selection screen. I added a route for this error, and read the exception to print out the actual error. This also means that you can navigate to that route and just end up with a generic/unknown error. The error is now displayed inside an ErrorReport around the button.

It also fixes the same when instantiating via a InstantiationButton from a stateless app. I wanted to rework this even more for stateless apps, as you might want to use a IInstantiationValidator to disallow some prefill values when creating a new instance with prefill (i.e. using the InstantiationButton with mapping). However, it doesn't seem like that's possible yet in the backend. If we get a feature request for that, we'd probably have to change both the backend and frontend at that point instead. Optimally the error should be displayed inline as a validation error in the stateless form, such that the user can make changes in the stateless form and retry - but right now I can't find a clear way for a change in the stateless form to make a difference in the IInstantiationValidator, so I'm scrapping that idea.

Update: We've been discussing alternative solutions here: https://digdir.slack.com/archives/C085N9FPDFS/p1744199886413489

Related Issue(s)

• closes Starting new instance from InstanceSelection doesn't render error messages #2578

Verification/QA

• Manual functionality testing
  • I have tested these changes manually
  • Creator of the original issue (or service owner) has been contacted for manual testing (or will be contacted when released in alpha)
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• Automated tests
  • Unit test(s) have been added/updated
  • Cypress E2E test(s) have been added/updated
  • No automatic tests are needed here (no functional changes/additions)
  • I want someone to help me make some tests
• UU/WCAG (follow these guidelines until we have our own)
  • I have tested with a screen reader/keyboard navigation/automated wcag validator
  • No testing done/necessary (no DOM/visual changes)
  • I want someone to help me perform accessibility testing
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• Support in Altinn Studio
  • Issue(s) created for support in Studio
  • This change/feature does not require any changes to Altinn Studio
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[Copilot]

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

test/e2e/pageobjects/party-mocks.ts:164

• Using the logical OR (||) here forces a falsy activeInstances value (specifically false) to default to an empty array, which may not be the intended behavior. Consider checking explicitly for a false value to ensure clarity in the mock behavior.

cy.intercept('**/active', whatToMock.activeInstances || []).as('activeInstances');

[sonarqubecloud]

Quality Gate failed

Failed conditions
37.3% Coverage on New Code (required ≥ 45%)
17.19% Condition Coverage on New Code (required ≥ 45%)

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