Reaction: Corrected handling of null beta list

src/thermophysicalModels/specie/reaction/Reactions/Reaction/Reaction.C

@@ -699,13 +699,16 @@ void Foam::Reaction<ReactionThermo>::dwdc
 
     // When third-body species are involved, additional terms are added
     // beta function returns an empty list when third-body are not involved
-    if (this->beta().size() > 0)
+    const List<Tuple2<label, scalar>>& beta = this->beta();
+    if (notNull(beta))
     {
-        scalarField dcidc(this->beta().size());
+        // This temporary array needs to be cached for efficiency
+        scalarField dcidc(beta.size());
         this->dcidc(p, T, c, dcidc);
-        forAll(this->beta(), j)
+
+        forAll(beta, j)
         {
-            label sj = this->beta()[j].first();
+            label sj = beta[j].first();
             sj = reduced ? c2s[sj] : sj;
             if (sj != -1)
             {