Update PopulateSwitchStatementHelpers.cs

src/Features/Core/Portable/PopulateSwitch/PopulateSwitchStatementHelpers.cs

@@ -53,7 +53,7 @@ public static ICollection<ISymbol> GetMissingEnumMembers(ISwitchOperation switch
                 // Check if the type of the expression is a nullable INamedTypeSymbol
                 // if the type is both nullable and an INamedTypeSymbol extact the type argument from the nullable
                 // and check if it is of enum type
-                switchExpression?.Type.IsNullable(out switchExpressionType);
+                switchExpressionType  = switchExpressionType.IsNullable(out underlyingType) ? underlyingType : switchExpressionType;
             }
 
             if (switchExpressionType?.TypeKind == TypeKind.Enum)