If \( X_1 = \begin{bmatrix} 1 \\ i \end{bmatrix} \), \( X_2 = \begin{bmatrix} i \\ 1 \end{bmatrix} \) are given vectors and
\( A = \begin{bmatrix} \cos \theta & \sin \theta \\ -\sin \theta & \cos \theta \end{bmatrix} \).
If \( P = [X_1, X_2] \), then \( P^{-1} A P \) is: