You have some js errors on this page( check the right left corner). The null exception is thrown, because you have a control on a page where you do not have for nulls and this result in an error. The Page object is null in edit mode, so if you use it somewhere this could be the reason. Another option is that you are running( have converted) .NET 2.0 or .NET 3.5 projects under(to) .NET 4.0.
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