[Expressive Design] Add null check for icon view in dropdown list.

- Fix crash when clicking spinner under original style.

Fix: 406403111
Test: disable flag & test manual
Flag: com.android.settingslib.widget.theme.flags.is_expressive_design_enabled
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:11d1c56f67295d2ecdaab3834e6174acdd22e0bb)
Merged-In: I4b2337ed0285afd943a9790d22089320e9a22b49
Change-Id: I4b2337ed0285afd943a9790d22089320e9a22b49