An error occurred on create a new pull request if base branch not existing

[ryz310]

See: Pulls - GitHub Docs

base
Required. The name of the branch you want the changes pulled into. This should be an existing branch on the current repository. You cannot submit a pull request to one repository that requests a merge to a base of another repository.

Should check base branch is exist or create a new branch.

GitHub APIを使ってブランチを新規作成する - ぷらすのブログ