revert to GetBlockByName "main"

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: Icefa594b25739ebaa331af2352bdcae1aaf81cf5
diff --git a/reference_model/src/main.cpp b/reference_model/src/main.cpp
index ccc65f9..0375a48 100644
--- a/reference_model/src/main.cpp
+++ b/reference_model/src/main.cpp
@@ -83,7 +83,7 @@
         FATAL_ERROR("Unable to load graph");
     }
 
-    SubgraphTraverser main_gt(tsh.GetMainRegion()->GetBlocks().front(), &tsh, nullptr);
+    SubgraphTraverser main_gt(tsh.GetMainRegion()->GetBlockByName("main"), &tsh, nullptr);
 
     if (main_gt.initializeGraph())
     {