Description

panel_regression.py has two bugs:

1. Lines 560 and 676 use # type: ignore[attr-defined], but mypy emits [union-attr] errors. The mismatched code means the ignores have no effect, and mypy fails during pre-commit.

2. plot_unit_effects (line 683) uses float(...) on an xarray result that can have a leftover treated_units dimension, causing TypeError: only 0-dimensional arrays can be converted to Python scalars. This makes test_panel_regression_plot_unit_effects fail on main (Python 3.14).

Root cause

1. Introduced in commit 1cf3387 (PR Add PanelRegression experiment class for fixed effects estimation #628). self.idata can be None, so accessing .posterior is a union-attr error, not attr-defined.
2. beta.sel(...).mean(dim=["chain", "draw"]) returns an array with a treated_units dimension, not a scalar.

Fix

1. Replace [attr-defined] with [union-attr] on lines 560 and 676.
2. Replace float(...) with .item() on line 683.