You can experience this error in case where UploadPaths property of RadFileExplorer is not set or is set to an empty array ( UploadPaths = new array). In such cases, the Upload control is not visible and the Upload property of RadFileExplorer returns null.
To avoid this you should modify apply modifications to the Upload control only during initial page load, for subsequent postbacks the settings will be applied from the viewstate.
RadFileExplorer1.Upload.MaxFileInputsCount = 1;
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