Migrating to .Net 4: Null reference exceptions thrown when adding event in xaml
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At my place of work we've recently been upgrading our codebase from .Net 3.5 to .Net 4 (C#). Most issues encountered have been solved however this one I can't figure out. We initialise controls and pages through a mix of xaml and code behind (based on developer preference), however one page is throwing *NullReferenceException* when opened. Here is a snippet of code that is (one of many controls) throwing. 

<B>All the code throwing exceptions is inside a DataTemplate</B> (*I figured that might be relevant*)

    <TextBox x:Name="Values" Grid.Column="1" Grid.Row="0" Margin="5,2,5,2"
      Text="{Binding ElementName=Descriptions, Path=SelectedValue, UpdateSourceTrigger=PropertyChanged,
      Mode=TwoWay,        
      Converter={StaticResource EmptyConverter}}"
      GotFocus="Column_GotFocus"
      MinWidth="100"
      CharacterCasing="Upper"
      Visibility="{Binding Path=IsValueVisible, Converter={StaticResource VisibilityConverter}}" 
    />

Now in this, the line throwing is:

    GotFocus="Column_GotFocus" 

Column_GotFocus is in the code behind file.
A few more facts: we had no issues before the migration, the exception gets throws continuously, and there are three different events causing this issue.

The three events throwing are: 

    GotFocus="Column_GotFocus"
    SelectionChanged="Descriptions_SelectionChanged"
    Click="Search_Click"

Removing these fixes our problem completely, but obviously causes functional problems with the software. Does anyone have any idea what could be causing these issues?