Streamline and improve the active drop search process

inventory.py

@@ -156,8 +156,11 @@ class DropsCampaign:
     def get_drop(self, drop_id: str) -> Optional[TimedDrop]:
         return self.timed_drops.get(drop_id)
 
-    def get_active_drop(self) -> Optional[TimedDrop]:
-        for drop in self.timed_drops.values():
-            if drop.can_earn:
-                return drop
-        return None
+    def get_active_drop(self) -> TimedDrop:
+        if not self.active:
+            raise RuntimeError("You can't call this for an unactive campaign")
+        drops = sorted(
+            (drop for drop in self.timed_drops.values() if drop.can_earn),
+            key=lambda d: d.remaining_minutes,
+        )
+        return drops[0]