fix(api): check shuttle presence before checking hopper (#18724)

# Overview
We should check the shuttle location before checking the hopper, so
we're not confusing the user by showing an empty hopper error when the
shuttle is obviously missing.
Fixes RQA-4291

Author: ahiuchingau

api/src/opentrons/hardware_control/modules/flex_stacker.py

@@ -418,11 +418,10 @@ async def dispense_labware(
     ) -> None:
         """Dispenses the next labware in the stacker."""
         self.verify_labware_height(labware_height)
+        await self._prepare_for_action()
         if enforce_hopper_lw_sensing:
             await self.verify_hopper_labware_presence(True)
 
-        await self._prepare_for_action()
-
         # Move platform along the X then Z axis
         await self._move_and_home_axis(StackerAxis.X, Direction.RETRACT, HOME_OFFSET_MD)
         await self._move_and_home_axis(StackerAxis.Z, Direction.EXTEND, HOME_OFFSET_SM)