Katalon : groovy.lang.MissingPropertyException : Using custom keyword in custom keyword

In Katalon, if I want to use CustomKeyword inside another CustomKeyword. I getting MissingPropertyException

Dialog Keywords

public class Dialog {

    @Keyword
    def clickCancel() {
      WebUI.click(findTestObject('Common/Components/Dialog/btn_Cancel'))
    }

    ...
}

Root Navigation Keywords

public class RootNavigations {

    @Keyword
    def checkDialogWorking() {
      WebUI.click(findTestObject('App/Page_Home/btn_OpenComparisons_Dialog_Home'))
      CustomKeywords.'com.app.Dialog.clickCancel'()
    }

    ...
}

Exception

Test Cases/Smoke Test/Application/Check Dialog 
FAILED because (of) (Stack trace: groovy.lang.MissingPropertyException: 
No such property: CustomKeywords for class: com.app.RootNavigations

import class into another Keyword class and declare its object to use that Keyword.

Root Navigation Keywords

public class RootNavigations {

    final dialog = new Dialog() // ******* imp step

    @Keyword
    def checkDialogWorking() {
      WebUI.click(findTestObject('App/Page_Home/btn_OpenComparisons_Dialog_Home'))
      this.dialog.clickCancel() // ******* imp step
    }

    ...
}