Fix using Box as variable in functional math

phi/geom/_box.py

@@ -278,6 +278,7 @@ def __init__(self, lower: Tensor = None, upper: Tensor = None, **size: Optional[
         self._upper = math.expand(self._upper, vector_shape)
         if self.size.vector.item_names is None:
             warnings.warn("Creating a Box without item names prevents certain operations like project()", DeprecationWarning, stacklevel=2)
+        self._shape = self._lower.shape & self._upper.shape
 
     def __getitem__(self, item):
         item = _keep_vector(slicing_dict(self, item))
@@ -320,9 +321,7 @@ def __variable_attrs__(self):
 
     @property
     def shape(self):
-        if self._lower is None or self._upper is None:
-            return None
-        return self._lower.shape & self._upper.shape
+        return self._shape
 
     @property
     def lower(self):