It's not clear to me how you created your kendo.custom but indeed, you must make sure to add all the files that combobox and validator depend on (and their dependencies in turn). It can be a tedious process to do this manually, but we have automated tools to help you.
You can use the Download Builder, available here: http://www.kendoui.com/custom-download
Another option is to use the Kendo UI Bootstrapper, which has a feature to generate a custom build containing only widgets that your code uses. https://github.com/kendo-labs/kendo-bootstrapper
Hope this helps.
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