Worktree dialog: selecting branch before checking "create worktree" reverts branch to default

In the worktree creation dialog, if you select a non-default branch first and then check the "create worktree" checkbox afterwards, the branch selection silently reverts to the repository's default branch.

Steps to reproduce:

1. Open the worktree/branch selection dialog
2. Select a non-default branch
3. Check the "create worktree" checkbox

Expected: Selected branch is preserved.
Actual: Branch selection resets to the default branch.

Workaround: Check the worktree checkbox first, then select the branch.