26920 fix exception call in fillet(wrong choice of face)

src/ChFi3d/ChFi3d_Builder_C1.cxx

@@ -2708,7 +2708,14 @@ void ChFi3d_Builder::PerformIntersectionAtEnd(const Standard_Integer Index)
             TopoDS_Edge aLocalEdge = edgesau;
             if (edgesau.Orientation() != orient)
               aLocalEdge.Reverse();
-            C2dint1 = BRep_Tool::CurveOnSurface(aLocalEdge,Face[0],ubid,vbid);
+            C2dint1 = BRep_Tool::CurveOnSurface(aLocalEdge, Face[0], ubid, vbid);
+
+            if (C2dint1.IsNull())
+            {
+              //std::swap(Face[0], facesau);
+              C2dint1 = BRep_Tool::CurveOnSurface(aLocalEdge, facesau, ubid, vbid);
+
+            }
           }
         }
         else {
@@ -2718,7 +2725,7 @@ void ChFi3d_Builder::PerformIntersectionAtEnd(const Standard_Integer Index)
 	//// for periodic 3d curves ////
 	if (cad.IsPeriodic())
 	{
-	  gp_Pnt2d P2d = BRep_Tool::Parameters( Vtx, Face[0] );
+          gp_Pnt2d P2d = BRep_Tool::Parameters(Vtx, facesau);
 	  Geom2dAPI_ProjectPointOnCurve Projector( P2d, C2dint1 );
 	  par = Projector.LowerDistanceParameter();
 	  Standard_Real shift = par-ParVtx;

tests/bugs/modalg_7/bug26920

@@ -0,0 +1,13 @@
+puts "========"
+puts "0026920: BRepBuidlerAPI_MakeFillet crash 2"
+puts "========"
+
+restore C:/WORK/opencascade/bugs/CR26920/bug26920.brep b1
+explode b1 e
+fillet result b1 0.25 b1_15
+
+checkprops result -s 1653.78
+checkshape result
+
+checknbshapes result -vertex 20 -edge 31 -wire 13 -face 13 -shell 1 -solid 1 -compsolid 0 -compound 1 -shape 80
+checkview -display result -2d -path ${imagedir}/${test_image}.png