fix: Order of geometry Id/material assignment in the DetectorBuilder (a…

…cts-project#3176)

Putting the `IMaterialDecorator` call after the geometry Id assignment, as the decorators (e.g. `JsonMaterialDecorator`) rely on the Id already being present in the hierarchy map.

Core/src/Detector/DetectorBuilder.cpp

@@ -37,16 +37,6 @@ Acts::Experimental::DetectorBuilder::construct(
 
   auto [volumes, portals, roots] = m_cfg.builder->construct(gctx);
 
-  // Decorate the volumes with material - Surface material only at this moment
-  if (m_cfg.materialDecorator != nullptr) {
-    ACTS_DEBUG("Decorating the detector with material");
-    std::for_each(volumes.begin(), volumes.end(), [&](auto& v) {
-      for (auto& sf : v->surfacePtrs()) {
-        m_cfg.materialDecorator->decorate(*sf);
-      }
-    });
-  }
-
   // Assign the geometry ids to the detector - if configured
   if (m_cfg.geoIdGenerator != nullptr) {
     ACTS_DEBUG("Assigning geometry ids to the detector");
@@ -57,6 +47,16 @@ Acts::Experimental::DetectorBuilder::construct(
     });
   }
 
+  // Decorate the volumes with material - Surface material only at this moment
+  if (m_cfg.materialDecorator != nullptr) {
+    ACTS_DEBUG("Decorating the detector with material");
+    std::for_each(volumes.begin(), volumes.end(), [&](auto& v) {
+      for (auto& sf : v->surfacePtrs()) {
+        m_cfg.materialDecorator->decorate(*sf);
+      }
+    });
+  }
+
   return Detector::makeShared(m_cfg.name, std::move(roots.volumes),
                               std::move(roots.volumeFinder));
 }