bpo-41611: IDLE: Catch TclError exceptions in AutoCompleteWindow.winconfig_event() (GH-26404)

Since the <Configure> event may occur after the
completion window is gone, catch potential
TclError exceptions when accessing acw.

(cherry picked from commit 4e2e5c1c4f792b4011e4c78d011baea2aee39f1b)