Hi Alexey,
Thank you for using Syncfusion products.
We are afraid that we were not able to reproduce the mentioned issue. This exception is handled when null value or empty string value or invalid style name is passed as argument to the method "paragraph.ApplyStyle()" and please ensure that the argument passed is valid. Please have a look at the sample in the below link and let us know if this helps.
http://files.syncfusion.com/support/DocIO.Windows/F90440/F90440.zipIf this issue still exists, please provide us with the code snippet or sample application, so that we can sort out the cause of the issue and provide you a solution.
Please let us know if you have any questions.
Regards,
Vijay