'Flutter: GoogleMapController? _controller vs Completer<GoogleMapController> _mapController?

im new to flutter and currently working on Google Maps and right now im creating floating action button for GoogleMaps parameter(zoom in-out). However when i was trying to pass zoom parameters to my floating button with simple GoogleMapController? _controller it wasn`t working when i tried it with the completer _mapController it worked, so whats the logic behind these two types?



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