Make sure that the returned grid is not None

`microgrid.grid.get()` returns `Grid | None`, so before accessing any
attributes from `grid` we need to make sure it is not `None`.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

tests/microgrid/test_grid.py

@@ -55,6 +55,7 @@ async def test_grid() -> None:
     microgrid.grid.initialize(components)
 
     grid = microgrid.grid.get()
+    assert grid is not None
 
     expected_fuse_current = Current.from_amperes(123.0)
     expected_fuse = Fuse(expected_fuse_current)