Fix mypy issue.

theater/controlpoint.py

@@ -18,7 +18,11 @@ from game import db
 from gen.ground_forces.combat_stance import CombatStance
 from .base import Base
 from .missiontarget import MissionTarget
-from .theatergroundobject import SamGroundObject, TheaterGroundObject
+from .theatergroundobject import (
+    BaseDefenseGroundObject,
+    SamGroundObject,
+    TheaterGroundObject,
+)
 
 if TYPE_CHECKING:
     from game import Game
@@ -52,7 +56,7 @@ class ControlPoint(MissionTarget):
         self.full_name = name
         self.at = at
         self.connected_objectives: List[TheaterGroundObject] = []
-        self.base_defenses: List[SamGroundObject] = []
+        self.base_defenses: List[BaseDefenseGroundObject] = []
 
         self.size = size
         self.importance = importance

theater/theatergroundobject.py

@@ -186,10 +186,14 @@ class MissileSiteGroundObject(TheaterGroundObject):
         )
 
 
+class BaseDefenseGroundObject(TheaterGroundObject):
+    """Base type for all base defenses."""
+
+
 # TODO: Differentiate types.
 # This type gets used both for AA sites (SAM, AAA, or SHORAD) but also for the
 # armor garrisons at airbases. These should each be split into their own types.
-class SamGroundObject(TheaterGroundObject):
+class SamGroundObject(BaseDefenseGroundObject):
     def __init__(self, name: str, group_id: int, position: Point,
                  control_point: ControlPoint, for_airbase: bool) -> None:
         super().__init__(
@@ -218,7 +222,7 @@ class SamGroundObject(TheaterGroundObject):
             return super().group_name
 
 
-class EwrGroundObject(TheaterGroundObject):
+class EwrGroundObject(BaseDefenseGroundObject):
     def __init__(self, name: str, group_id: int, position: Point,
                  control_point: ControlPoint) -> None:
         super().__init__(