Reset the viewer to None after closing the environment.

Author: amacati

crazyflow/sim/sim.py

@@ -280,6 +280,7 @@ def seed(self, seed: int):
     def close(self):
         if self.viewer is not None:
             self.viewer.close()
+        self.viewer = None
 
     @property
     def time(self) -> Array: