Can you give us information exactly what version of Sitefinity and SDK have you tried using together and has the project been upgraded from previous version. Also, please double-check if the assembly references are not the old ones and if so, update them.
Also, please try reproducing this on an empty project and send us the exact steps for reproduction, because we tried reproducing this bug with different Sitefinity - SDK combination of versions and no issue appeared.
the Telerik team
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