check for IsMultithreadedApplication, add guard for no SD code attached

src/g4integration/geometry/AdePTGeometryBridge.cpp

@@ -432,6 +432,21 @@ void AdePTGeometryBridge::InitVolAuxData(adeptint::VolAuxData *volAuxData, G4Hep
   vecgeom::NavigationState rootNavState;
   visitGeometry(g4world, vecgeomWorld, rootNavState);
 
+  // Defensive check for attached SD code: GetSensitiveDetector() is thread local.
+  // If the GPU geometry is initialized on a thread without SD code attached, issue a warning.
+  {
+    const std::size_t nLVs = vecgeom::GeoManager::Instance().GetRegisteredVolumesCount();
+    std::size_t nSensitive = 0;
+    for (std::size_t i = 0; i < nLVs; ++i) {
+      if (volAuxData[i].fSensIndex >= 0) ++nSensitive;
+    }
+    if (nSensitive == 0) {
+      G4Exception("AdePTGeometryBridge::InitVolAuxData", "NoSensitiveVolumes", JustWarning,
+                  "No logical volume is marked sensitive: AdePT was likely initialized on a thread without attached "
+                  "sensitive detectors; GPU energy deposits will not be scored.");
+    }
+  }
+
   auto findRegionName = [](int rid) -> std::string {
     for (auto *r : *G4RegionStore::GetInstance()) {
       if (r && r->GetInstanceID() == rid) return r->GetName();

src/g4integration/tracking_managers/AdePTTrackingManager.cc

@@ -247,10 +247,13 @@ void AdePTTrackingManager::InitializeAdePT()
 {
   auto tid = G4Threading::G4GetThreadId();
 
-  // Master thread cannot initialize AdePT as the number of G4 worker threads may not yet be known in applications.
+  // Master thread must not initialize AdePT as the number of G4 worker threads may not yet be known in applications.
+  // Also sensitive detectors are attached per worker thread and may be missed on the master.
   // In sequential mode, the tid is -2 in most builds, but some sequential Geant4 builds return MASTER_ID (-1).
-  // Use GetMasterRunManager() to distinguish an actual MT master thread from sequential mode.
-  if (tid == G4Threading::MASTER_ID && G4MTRunManager::GetMasterRunManager() != nullptr) return;
+  // G4MTRunManager::GetMasterRunManager() cannot be used to detect an MT master: applications like CMSSW
+  // drive a G4MTRunManagerKernel directly without ever creating a G4MTRunManager. IsMultithreadedApplication()
+  // is set by the G4MTRunManagerKernel constructor and therefore covers both cases.
+  if (tid == G4Threading::MASTER_ID && G4Threading::IsMultithreadedApplication()) return;
 
   // a condition variable and a mutex is used for the initialization:
   // The first G4 worker that reaches the initialization, needs to initialize AdePT.