Fix issue where code segfaults when surface material is missing

Ignores missing material and warns user of bad FBX

Only edge case is handled while previously working models should operate as before

src/fbx/materials/FbxMaterials.cpp

@@ -12,6 +12,8 @@
 #include "RoughnessMetallicMaterials.hpp"
 #include "TraditionalMaterials.hpp"
 
+static int warnMtrCount = 0;
+
 FbxMaterialsAccess::FbxMaterialsAccess(
     const FbxMesh* pMesh,
     const std::map<const FbxTexture*, FbxString>& textureLocations)
@@ -43,9 +45,16 @@ FbxMaterialsAccess::FbxMaterialsAccess(
       continue;
     }
 
-    FbxSurfaceMaterial* surfaceMaterial =
+    auto* surfaceMaterial =
         mesh->GetNode()->GetSrcObject<FbxSurfaceMaterial>(materialNum);
 
+    if (!surfaceMaterial) {
+      if (++warnMtrCount == 1) {
+        fmt::printf("Warning: Reference to missing surface material.\n");
+        fmt::printf("         (Further warnings of this type squelched.)\n");
+      }
+    }
+
     if (materialNum >= summaries.size()) {
       summaries.resize(materialNum + 1);
     }
@@ -57,7 +66,8 @@ FbxMaterialsAccess::FbxMaterialsAccess(
     if (materialNum >= userProperties.size()) {
       userProperties.resize(materialNum + 1);
     }
-    if (userProperties[materialNum].empty()) {
+    if (surfaceMaterial && userProperties[materialNum].empty()) {
+
       FbxProperty objectProperty = surfaceMaterial->GetFirstProperty();
       while (objectProperty.IsValid()) {
         if (objectProperty.GetFlag(FbxPropertyFlags::eUserDefined)) {
@@ -97,6 +107,9 @@ const std::vector<std::string> FbxMaterialsAccess::GetUserProperties(const int p
 std::unique_ptr<FbxMaterialInfo> FbxMaterialsAccess::GetMaterialInfo(
     FbxSurfaceMaterial* material,
     const std::map<const FbxTexture*, FbxString>& textureLocations) {
+  if (!material) {
+    return nullptr;
+  }
   std::unique_ptr<FbxMaterialInfo> res =
       FbxStingrayPBSMaterialResolver(material, textureLocations).resolve();
   if (res == nullptr) {