I'm trying educate myself, so I'm working on tracking down an error I have on one of my umbraco sites. I downloaded the source and am trying to track down where exactly the error is in the code so I'll know what to fix.

This is a total beginner question, but coming from other languages the stack trace line numbers don't make sense. For example, in visual studio the following method begin on line 294. However it seems to indicate that the exception ocurred on line 54. What's going on here?

umbraco.controls.ContentControl.addControlNew(Property p, TabPage tp, String Caption) +54

Here's the full stack

[NullReferenceException: Object reference not set to an instance of an object.]
   umbraco.controls.ContentControl.addControlNew(Property p, TabPage tp, String Caption) +54
   umbraco.controls.ContentControl.CreateChildControls() +561
   System.Web.UI.Control.EnsureChildControls() +102
   umbraco.controls.ContentControl.OnInit(EventArgs e) +32
   System.Web.UI.Control.InitRecursive(Control namingContainer) +140
   System.Web.UI.Control.AddedControl(Control control, Int32 index) +197
   System.Web.UI.ControlCollection.Add(Control child) +79
   umbraco.cms.presentation.editContent.OnInit(EventArgs e) +1250
   System.Web.UI.Control.InitRecursive(Control namingContainer) +140
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +480