[xbuild] Don't crash when reporting top level unknown elements

diff --git a/mcs/class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine/Project.cs b/mcs/class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine/Project.cs
index c22b8c749c916f44f4fecba2b1d3501143129ef4..6ac858e528fbf2cc5c25edb55b13bf49d71e5a41 100644 (file)
--- a/mcs/class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine/Project.cs
+++ b/mcs/class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine/Project.cs
@@ -951,7 +951,8 @@ namespace Microsoft.Build.BuildEngine {
                                                AddChoose (xe, ip);
                                                break;
                                        default:
-                                               throw new InvalidProjectFileException (String.Format ("Invalid element '{0}' in project file '{1}'.", xe.Name, ip.FullFileName));
+                                               var pf = ip == null ? null : string.Format (" '{0}'", ip.FullFileName);
+                                               throw new InvalidProjectFileException (String.Format ("Invalid element '{0}' in project file{1}.", xe.Name, pf));
                                        }
                                }
                        }