Object reference not set to an instance of an object.

[sybaris]

Describe the bug
An exception "Object reference not set to an instance of an object" is thrown when using a particular fetchxml

To Reproduce
The following code reproduce the issue

[TestMethod]
       public void TestIssue()
       {
           string fetchxml = @"
               <fetch xmlns:generator='MarkMpn.SQL4CDS'>
                 <entity name='solutioncomponent'>

                   <attribute name='solutionid' />

                   <link-entity name='solutioncomponentdefinition' to='componenttype' from='objecttypecode' alias='solutioncomponentdefinition' link-type='outer'>
                     <attribute name='name' alias='solutioncomponentdefinitionname'/>
                     <attribute name='solutioncomponentdefinitionid' />
                   </link-entity>
                   <filter>
                
<condition attribute = 'name' entityname='solutioncomponentdefinition' operator='not-in'>
<value>AppSetting</value>
<value>AppElement</value>
</condition>

                   </filter>
                 </entity>
               </fetch>
               
";

           var context = MiddlewareBuilder
           .New()
           .AddCrud()
           .AddFakeMessageExecutors()
           .AddFakeMessageExecutors(Assembly.GetAssembly(typeof(RetrieveVersionRequestExecutor)))
           .AddGenericFakeMessageExecutors(Assembly.GetAssembly(typeof(NavigateToNextEntityOrganizationRequestExecutor)))
           .UseCrud()
           .UseMessages()
           .SetLicense(FakeXrmEasyLicense.RPL_1_5)
           .Build();
           context.EnableProxyTypes(Assembly.GetExecutingAssembly());
           context.InitializeMetadata(Assembly.GetExecutingAssembly());

           context.GetOrganizationService().RetrieveMultiple(new FetchExpression(fetchxml));
       }

Expected behavior
No exception, and get the result
Note that if you remove the condition block in the xml part, the unit test is green...

** FakeXrmEasy and Dynamics 365 / CRM version **
2.3.3