PR description lost if base branch gets merged

[LeonBew]

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When creating a new PR on a branch that is targeting a non-default base branch (not main), if that base branch gets merged/deleted while still writing the PR description in the creation form, the page throws "branchname does not exist" and resets the entire PR description form, in my case wiping the description back to a PR description template.

This error and the loss of the description happen before the PR is submitted, after filling it out, and clicking "Create Pull Request" / "Draft Pull Request".

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