OrderBy null reference error.

Hi, I'm new in all of this about LinQ and Entity Framework.

Here is the code:

public class MyClass
 { 
   public int MyClassId{get;set;} 
   public string Description{get;set;} 

   public virtual iList<ChildClass> Childs {get;set;} 
}
public class ChildClass 
{
   public int ChildClassId {get;set;}
   public string Description {get;set;}

   public int MyClassId {get;set;} 
   public virtual MyClass MyClass {get;set;}
} 
....
IQueryable<MyClass> col= from m in context.MyClass 
                              select m;
if (condition) 
{ 
   col = col.Include(i=>i.Childs);
}
//PropertyName is an string argument of the method
var propertyInfo = new MyClass().GetType().GetProperty("PropertyName");
 if (!(propertyInfo == null))
{
   if (orderDesc) 
   {
      col= col.OrderByDescending(p => propertyInfo.GetValue(p));
   } 
   else
   {
      col = col.OrderBy(p => propertyInfo.GetValue(p));
   } 
} 
return await col.ToListAsync(); 

The problem is when the condition in the first if clause is true and add the .include childs to col, and then try to applies the orderby y get the following error:

If I Comment the first if clause (where adds the childs) its works and retrieve me the collection with no error.  Also I noticed that if I change

p => propertyInfo.GetValue(p)

with for example

p => p.Description

works fine wihout comment the include.

Any suggestion?

Thanks!