System.NullReferenceException when basing template from mast

I'm having an issue with a site that I have upgraded from 3.7 sp4 to 6.3.5000. 

I've got a 'standard.master' that was a template imported from a 3.7 sp4 project. The 'standard.master' file is also based off of a 'base.master'. 

What I'm doing is trying to base a template which is originally based off standard, to another template based off standard. My template section loads properly, and pretty much functions ok except when trying to do the 'base of template' function. I haven't fully cleaned the files so I'm guessing I got a bit to do.

Anything I should consider?

Error log stack trace

 Message: HandlingInstanceID: 354cb7a3-94e0-4177-8e91-51316a74924c
An exception of type 'System.NullReferenceException' occurred and was caught.
-----------------------------------------------------------------------------
09/25/2014 17:18:23
Type : System.NullReferenceException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Message : Object reference not set to an instance of an object.
Source : Telerik.Sitefinity
Help link : 
Data : System.Collections.ListDictionaryInternal
TargetSite : TDraft GetTemp(TDraft)
HResult : -2147467261
Stack Trace :    at Telerik.Sitefinity.Lifecycle.LifecycleDecorator`2.GetTemp(TDraft draftItem)
   at Telerik.Sitefinity.Lifecycle.LifecycleDecorator`2.CheckOut(TDraft masterItem, CultureInfo culture)
   at Telerik.Sitefinity.Modules.Pages.Web.Services.PageTemplatesService.ChangeTemplateInternal(String templateId, String newTemplateId)
   at Telerik.Sitefinity.Modules.Pages.Web.Services.PageTemplatesService.ChangeTemplate(String templateId, String newTemplateId)
   at SyncInvokeChangeTemplate(Object , Object[] , Object[] )
   at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs)
   at System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage5(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage41(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage31(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage3(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage2(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage11(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage1(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean isOperationContextSet)

Additional Info:

MachineName : M1730
TimeStamp : 9/25/2014 9:18:23 PM
FullName : Telerik.Sitefinity.Utilities, Version=6.3.5000.0, Culture=neutral, PublicKeyToken=b28c218413bdf563
AppDomainName : bf4e19f0-1-130561533731843553
ThreadIdentity : 
WindowsIdentity : SMDHU\crezsonya
Requested URL : localhost:52153/.../


Category: ErrorLog

Priority: 0

EventId: 90000

Severity: Error

Title:Enterprise Library Exception Handling

Machine: M1730

App Domain: bf4e19f0-1-130561533731843553

ProcessId: 8968

Process Name: C:\Program Files (x86)\Common Files\Microsoft Shared\DevServer\11.0\WebDev.WebServer40.exe

Thread Name: 

Win32 ThreadId:7256

Extended Properties: 
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So it was exactly that. The templates I was attempting to 'rebase' had controls/widgets that weren't found... so I removed the widgets not found and the 'rebasing' the template worked.