I tried to reproduce the described problem by attaching unmodified external dialogs to a RadEditor instance with the latest release of RadControls, but to no avail. I have included my test sample to this post. Please compare it to your actual project and check if there are differences in the setup. Here
you can examine the behavior on my end.
It is possible that the error stems from changes in the external dialogs. I would suggest trying the same steps with unmodified dialogs and check if the problem persists. Also, please ensure that the external dialogs and the RadControls assemblies are from the same release.
If you are still having difficulties after examining the information above, please try changing the attached sample in order to recreate your scenario. In case this is not possible, you can also send a simple, fully runnable project that isolates the problem so that I can inspect it locally.
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