diff --git a/geom/gdml/inc/TGDMLWrite.h b/geom/gdml/inc/TGDMLWrite.h
index 4210b208d4358..b8cafe6e1865d 100644
--- a/geom/gdml/inc/TGDMLWrite.h
+++ b/geom/gdml/inc/TGDMLWrite.h
@@ -126,6 +126,7 @@ class TGDMLWrite : public TObject {
    XMLDocPointer_t  fGdmlFile;                             //pointer storing xml file
    TString fDefault_lunit;                                 //Default unit of length (depends on ROOT unit system)
    TString fTopVolumeName;                                 //name of top volume
+   TGeoVolume *fTopVolume = nullptr;                       //top volume of the tree being written
    TXMLEngine *fGdmlE;                                     //xml engine pointer
 
    XMLNodePointer_t fDefineNode;                           //main <define> node...
diff --git a/geom/gdml/src/TGDMLParse.cxx b/geom/gdml/src/TGDMLParse.cxx
index dce3c50b1d01d..9a41e06673a40 100644
--- a/geom/gdml/src/TGDMLParse.cxx
+++ b/geom/gdml/src/TGDMLParse.cxx
@@ -1681,6 +1681,8 @@ TGeoTranslation *TGDMLParse::GetPosition(const char *name)
    if (!pos && fposmap.find(name) != fposmap.end())
       pos = fposmap[name];
 
+   if (!pos)
+      Error("GetPosition", "Position %s not defined", name);
    return pos;
 }
 
@@ -1698,6 +1700,8 @@ TGeoRotation *TGDMLParse::GetRotation(const char *name)
    if (!rot && frotmap.find(name) != frotmap.end())
       rot = frotmap[name];
 
+   if (!rot)
+      Error("GetRotation", "Rotation %s not defined", name);
    return rot;
 }
 
@@ -1715,6 +1719,8 @@ TGeoScale *TGDMLParse::GetScaleObj(const char *name)
    if (!scl && fsclmap.find(name) != fsclmap.end())
       scl = fsclmap[name];
 
+   if (!scl)
+      Error("GetScale", "Scale %s not defined", name);
    return scl;
 }
 
@@ -1732,6 +1738,8 @@ TGeoShape *TGDMLParse::GetSolid(const char *name)
    if (!sol && fsolmap.find(name) != fsolmap.end())
       sol = fsolmap[name];
 
+   if (!sol)
+      Error("GetSolid", "Solid %s not defined", name);
    return sol;
 }
 
@@ -1749,6 +1757,8 @@ TGeoVolume *TGDMLParse::GetVolume(const char *name)
    if (!vol && fvolmap.find(name) != fvolmap.end())
       vol = fvolmap[name];
 
+   if (!vol)
+      Error("GetVolume", "Volume %s not defined", name);
    return vol;
 }
 
diff --git a/geom/gdml/src/TGDMLWrite.cxx b/geom/gdml/src/TGDMLWrite.cxx
index dcbfbdd570912..b18089b0868f2 100644
--- a/geom/gdml/src/TGDMLWrite.cxx
+++ b/geom/gdml/src/TGDMLWrite.cxx
@@ -240,7 +240,8 @@ void TGDMLWrite::WriteGDMLfile(TGeoManager * geomanager, const char* filename, T
     Info("WriteGDMLfile", "Top volume does not exist!");
     return;
   }
-  fTopVolumeName = "";
+  fTopVolume = node->GetVolume();
+  fTopVolumeName = fTopVolume->GetName();
   WriteGDMLfile(geomanager, node, materials, filename, option);
 }
 
@@ -259,6 +260,7 @@ void TGDMLWrite::WriteGDMLfile(TGeoManager * geomanager, TGeoNode* node, const c
   for(TGeoMaterial* m : extract.materials)
     materials.Add(m);
   fTopVolumeName = volume->GetName();
+  fTopVolume = volume;
   fSurfaceList.clear();
   fVolumeList.clear();
   fNodeList.clear();
@@ -569,7 +571,7 @@ void TGDMLWrite::ExtractVolumes(TGeoNode* node)
    fNodeList.insert(node);
    fVolumeList.insert(volume);
    //create the name for volume/assembly
-   if (volume->IsTopVolume()) {
+   if (volume == fTopVolume) {
       //not needed a special function for generating name
       volname = volume->GetName();
       fTopVolumeName = volname;
@@ -2432,6 +2434,7 @@ void TGDMLWrite::WriteGDMLfile(TGeoManager * geomanager, TGeoVolume* volume, con
   for(TGeoMaterial* m : extract.materials)
     materials.Add(m);
   fTopVolumeName = volume->GetName();
+  fTopVolume = volume;
   fSurfaceList.clear();
   fVolumeList.clear();
   fNodeList.clear();
@@ -2583,7 +2586,7 @@ void TGDMLWrite::ExtractVolumes(TGeoVolume* volume)
    const TString fltPrecision = TString::Format("%%.%dg", fFltPrecision);
 
    //create the name for volume/assembly
-   if (volume->IsTopVolume()) {
+   if (volume == fTopVolume) {
       //not needed a special function for generating name
       volname = volume->GetName();
       fTopVolumeName = volname;
diff --git a/geom/geom/src/TGeoManager.cxx b/geom/geom/src/TGeoManager.cxx
index 384f9f1c9c745..e92fc67212183 100644
--- a/geom/geom/src/TGeoManager.cxx
+++ b/geom/geom/src/TGeoManager.cxx
@@ -3820,13 +3820,13 @@ TGeoManager *TGeoManager::Import(const char *filename, const char *name, Option_
 {
    if (fgLock) {
       ::Warning("TGeoManager::Import", "TGeoMananager in lock mode. NOT IMPORTING new geometry");
-      return NULL;
+      return nullptr;
    }
    if (!filename) return 0;
    if (fgVerboseLevel>0) ::Info("TGeoManager::Import","Reading geometry from file: %s",filename);
 
    if (gGeoManager) delete gGeoManager;
-   gGeoManager = 0;
+   gGeoManager = nullptr;
 
    if (strstr(filename,".gdml")) {
       // import from a gdml file
@@ -3834,8 +3834,10 @@ TGeoManager *TGeoManager::Import(const char *filename, const char *name, Option_
       TString cmd = TString::Format("TGDMLParse::StartGDML(\"%s\")", filename);
       TGeoVolume* world = (TGeoVolume*)gROOT->ProcessLineFast(cmd);
 
-      if(world == 0) {
-         ::Error("TGeoManager::Import", "Cannot open file");
+      if(world == nullptr) {
+         delete gGeoManager;
+         gGeoManager = nullptr;
+         ::Error("TGeoManager::Import", "Cannot read file %s", filename);
       }
       else {
          gGeoManager->SetTopVolume(world);
@@ -3847,12 +3849,12 @@ TGeoManager *TGeoManager::Import(const char *filename, const char *name, Option_
       TDirectory::TContext ctxt;
       // in case a web file is specified, use the cacheread option to cache
       // this file in the cache directory
-      TFile *f = 0;
+      TFile *f = nullptr;
       if (strstr(filename,"http")) f = TFile::Open(filename,"CACHEREAD");
       else                         f = TFile::Open(filename);
       if (!f || f->IsZombie()) {
          ::Error("TGeoManager::Import", "Cannot open file");
-         return 0;
+         return nullptr;
       }
       if (name && strlen(name) > 0) {
          gGeoManager = (TGeoManager*)f->Get(name);
diff --git a/geom/geom/src/TGeoMatrix.cxx b/geom/geom/src/TGeoMatrix.cxx
index 6a34ed9362129..cb196b48b656d 100644
--- a/geom/geom/src/TGeoMatrix.cxx
+++ b/geom/geom/src/TGeoMatrix.cxx
@@ -683,6 +683,7 @@ TGeoHMatrix TGeoTranslation::Inverse() const
 {
    TGeoHMatrix h;
    h = *this;
+   h.ResetBit(kGeoRegistered);
    Double_t tr[3];
    tr[0] = -fTranslation[0];
    tr[1] = -fTranslation[1];
@@ -977,6 +978,7 @@ TGeoHMatrix TGeoRotation::Inverse() const
 {
    TGeoHMatrix h;
    h = *this;
+   h.ResetBit(kGeoRegistered);
    Double_t newrot[9];
    newrot[0] = fRotationMatrix[0];
    newrot[1] = fRotationMatrix[3];
@@ -1527,6 +1529,7 @@ TGeoHMatrix TGeoScale::Inverse() const
 {
    TGeoHMatrix h;
    h = *this;
+   h.ResetBit(kGeoRegistered);
    Double_t scale[3];
    scale[0] = 1./fScale[0];
    scale[1] = 1./fScale[1];
@@ -1826,6 +1829,7 @@ TGeoHMatrix TGeoCombiTrans::Inverse() const
 {
    TGeoHMatrix h;
    h = *this;
+   h.ResetBit(kGeoRegistered);
    Bool_t is_tr = IsTranslation();
    Bool_t is_rot = IsRotation();
    Double_t tr[3];
@@ -2232,6 +2236,7 @@ void TGeoGenTrans::SetScale(Double_t sx, Double_t sy, Double_t sz)
 TGeoHMatrix TGeoGenTrans::Inverse() const
 {
    TGeoHMatrix h = *this;
+   h.ResetBit(kGeoRegistered);
    return h;
 }
 
@@ -2458,6 +2463,7 @@ TGeoHMatrix TGeoHMatrix::Inverse() const
 {
    TGeoHMatrix h;
    h = *this;
+   h.ResetBit(kGeoRegistered);
    if (IsTranslation()) {
       Double_t tr[3];
       tr[0] = -fTranslation[0]*fRotationMatrix[0] - fTranslation[1]*fRotationMatrix[3] - fTranslation[2]*fRotationMatrix[6];