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Description

When planning changes on a linux function app slot that has been previously created with an older version of the provider, the plugin panics during the migration of the model.
This prevents from changes in the whole terraform workspace, so may be blocking for the many users in the same situation.

Stack trace from the terraform-provider-azurerm_v3.101.0_x5 plugin:

panic: interface conversion: interface {} is nil, not string

goroutine 57 [running]:
github.com/hashicorp/terraform-provider-azurerm/internal/services/appservice/migration.(*LinuxFunctionAppSlotV0toV1).UpgradeFunc.LinuxFunctionAppSlotV0toV1.UpgradeFunc.func1({0x0?, 0x0?}, 0x0?, {0x0?, 0x0?})
...
panic: interface conversion: interface {} is nil, not string

The error is caused by unsafe get of the service_plan_id key from the rawState map during state migration from v0 to v1.

PR Checklist

• I have followed the guidelines in our Contributing Documentation.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
• I have updated/added Documentation as required written in a helpful and kind way to assist users that may be unfamiliar with the resource / data source.
• I have used a meaningful PR title to help maintainers and other users understand this change and help prevent duplicate work.
  For example: “resource_name_here - description of change e.g. adding property new_property_name_here”

Changes to existing Resource / Data Source

• I have added an explanation of what my changes do and why I'd like you to include them (This may be covered by linking to an issue above, but may benefit from additional explanation).
• I have written new tests for my resource or datasource changes & updated any relevent documentation.
• I have successfully run tests with my changes locally. If not, please provide details on testing challenges that prevented you running the tests.
• (For changes that include a state migration only). I have manually tested the migration path between relevant versions of the provider.

The test has been taken creating the resource with the 3.87.0 version of the provider and planned again with 3.101.0 version. No plan edits returned.

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_linux_function_app_slot - fixed error when resource is firstly deployed with provider version <= 3.88.0 and then upgraded

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

Fixes #0000