Bonsai: guard decorator draws against None and empty lists

Three crashes that surfaced when viewport decorators ran against
selected non-IFC blender objects or top-level objects with no
aggregate parent:

- WallAxisDecorator.draw_wall_axis: tool.Ifc.get_entity(obj) returns
  None for a non-IFC selection (default cube, lamp, camera). The
  subsequent element.is_a("IfcWall") raised AttributeError on every
  redraw. Guard with `element and element.is_a(...)`.
- _ConnectedNetworkPathDecorator flow-segment loop: same shape;
  iterates entries that may be None, calls .is_a("IfcFlowSegment")
  unconditionally. Same guard.
- AggregateDecorator.draw_aggregate: indexes aggregates_list[-1]
  unconditionally in the else branch; raises IndexError when the
  selected element has no aggregate parent. Also leaves `aggregate`
  unbound across loop iterations in the `in_aggregate_mode` branch
  when `index <= 0`. Define `aggregate = None` per loop iteration
  and guard the [-1] indexing with `elif aggregates_list:`.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/aggregate/decorator.py

@@ -158,12 +158,13 @@ class AggregateDecorator(tool.Blender.ViewportDecorator):
                 aggregates.append(obj)
                 continue
 
+            aggregate = None
             aggregates_list = tool.Aggregate.get_aggregates_recursively(element)
             if props.in_aggregate_mode and props.editing_aggregate:
                 index = aggregates_list.index(tool.Ifc.get_entity(props.editing_aggregate))
                 if index > 0:
                     aggregate = aggregates_list[index - 1]
-            else:
+            elif aggregates_list:
                 aggregate = aggregates_list[-1]
             if aggregate:
                 aggregates.append(tool.Ifc.get_object(aggregate))

src/bonsai/bonsai/bim/module/model/decorator.py

@@ -1597,7 +1597,7 @@ class WallAxisDecorator(tool.Blender.ViewportDecorator):
         self.line_shader.uniform_float("lineWidth", 2.0)
         for obj in context.selected_objects:
             element = tool.Ifc.get_entity(obj)
-            if element.is_a("IfcWall"):
+            if element and element.is_a("IfcWall"):
                 layers = tool.Model.get_material_layer_parameters(element)
                 axis = tool.Model.get_wall_axis(obj, layers)
                 side = [tuple(list(v) + [obj.location.z]) for v in axis["side"]]
@@ -2702,7 +2702,7 @@ class MEPSystemPathDecorator(_ConnectedNetworkPathDecorator):
         lines: list[tuple[tuple[float, float, float], tuple[float, float, float]]] = []
         port_positions: list[tuple[float, float, float]] = []
         for element in connected:
-            if element.is_a("IfcFlowSegment"):
+            if element and element.is_a("IfcFlowSegment"):
                 if not tool.Geometry.has_axis_representation(element):
                     continue
                 obj = tool.Ifc.get_object(element)