Fix T89982: Geometry Nodes: 'New' Button tries to create node_tree on active modifier, rather than button context

When done from the Properties Editor, the context's modifier should be
used (this is where the button is located), when done from elsewhere,
the active modifier is still the way to go (since the context modifier is
not available then)

Maniphest Tasks: T89982

Differential Revision: https://developer.blender.org/D11972

release/scripts/startup/bl_operators/geometry_nodes.py

@@ -81,7 +81,10 @@ class NewGeometryNodeTreeAssign(Operator):
         return geometry_modifier_poll(context)
 
     def execute(self, context):
-        modifier = context.object.modifiers.active
+        if context.area.type == 'PROPERTIES':
+            modifier = context.modifier
+        else:
+            modifier = context.object.modifiers.active
 
         if not modifier:
             return {'CANCELLED'}