[RF] Make tutorial r619 runnable without clad.

Also fix a warning that protects from a Gaussian with sigma=0.

Author: hageboeck

tutorials/roofit/roofit/rf619_discrete_profiling.C

@@ -35,7 +35,7 @@ void rf619_discrete_profiling()
 
    // Category 1 Pdf-s: Gaussian + Landau.
    RooRealVar mean("mean", "shared mean", 25, 0, 50);
-   RooRealVar sigmaG("sigmaG", "Gaussian width", 2.0, 0.0, 5.0);
+   RooRealVar sigmaG("sigmaG", "Gaussian width", 2.0, 0.01, 5.0);
    RooRealVar sigmaL("sigmaL", "Landau width", 3.0, 1.0, 8.0);
 
    RooGaussian gauss1("gauss1", "Gaussian", x, mean, sigmaG);
@@ -118,8 +118,16 @@ void rf619_discrete_profiling()
       data->add(vars);
    }
 
-   // Create NLL with codegen and minimize it via the discrete profiling method.
-   std::unique_ptr<RooAbsReal> nll1(simPdf.createNLL(*data, EvalBackend("codegen")));
+   // Create an NLL and minimize it via the discrete profiling method.
+   std::unique_ptr<RooAbsReal> nll1;
+   try {
+      // Try using an NLL with automatic differentiation if available in ROOT
+      nll1.reset(simPdf.createNLL(*data, EvalBackend("codegen")));
+   } catch (std::runtime_error &e) {
+      std::cout << "Automatic differentiation not activated, falling back to numeric mode (" << e.what() << ")\n";
+      // Fallback to numeric differentiation
+      nll1.reset(simPdf.createNLL(*data));
+   }
    RooMinimizer minim(*nll1);
 
    minim.setStrategy(1);