fix img3d

GUI/View/Realspace/RealspaceBuilder.cpp

@@ -105,6 +105,9 @@ std::unique_ptr<const double2d_t> layerRoughnessMap(const LayerItem& layerItem,
         autocorrelation = std::make_unique<K_CorrelationModel>(
             br->sigma().dVal(), br->hurst().dVal(), br->lateralCorrelationLength().dVal());
 
+    if (!autocorrelation)
+        return result;
+
     auto roughness = LayerRoughness(autocorrelation.get(), interlayer.get());
     if (roughness.sigma() == 0)
         return result;