Workaround for issue #3160

Author: vweijsters

StyleCop.Analyzers/StyleCop.Analyzers/OrderingRules/SA1201ElementsMustAppearInTheCorrectOrder.cs

@@ -296,7 +296,11 @@ private static void HandleMemberList(SyntaxNodeAnalysisContext context, Immutabl
 
                 if (index > nextIndex)
                 {
-                    context.ReportDiagnostic(Diagnostic.Create(Descriptor, NamedTypeHelpers.GetNameOrIdentifierLocation(members[i + 1]), MemberNames[nextElementSyntaxKind], MemberNames[elementSyntaxKind]));
+                    // [Issue #3160] Added hardening here to make sure that this won't crash when working with invalid code.
+                    var nextElementMemberName = MemberNames.ContainsKey(nextElementSyntaxKind) ? MemberNames[nextElementSyntaxKind] : "<unknown>";
+                    var elementMemberName = MemberNames.ContainsKey(elementSyntaxKind) ? MemberNames[elementSyntaxKind] : "<unknown>";
+
+                    context.ReportDiagnostic(Diagnostic.Create(Descriptor, NamedTypeHelpers.GetNameOrIdentifierLocation(members[i + 1]), nextElementMemberName, elementMemberName));
                 }
             }
         }