This example will perform validation on whitespaces and empty lines through the use of stripHTML, stripping all HTML tags before performing validation. This will ensure reliable and accurate validation as empty spaces and white spaces are interpreted as "<p><br></p> " and "<p> <p>" by the Rich Text Editor. You may change the following code to perform various validation accordingly for example: 

Length Limitations

Prohibited Content

Content Sanitization

and many more...

import java.util.Arrays;
import org.joget.apps.app.service.AppUtil;
import org.joget.apps.form.model.Element;
import org.joget.apps.form.model.FormData;
import org.joget.apps.form.service.FormUtil;

public boolean validate(Element element, FormData formData, String[] values) {
    boolean result = true;
    
    for (String value : values) {
        // Check if editor input is null or is empty
        System.out.println("Stripped: " + stripHtml(value));
        if(value != null && (stripHtml(value).equals(" ") || stripHtml(value).trim().isEmpty())){
            String id = FormUtil.getElementParameterName(element);
			//This will display the custom error message
			//Customize it accordingly	
            formData.addFormError(id, "ILLEGAL VALUE");
            result = false;
        }
        System.out.println(value.trim());
    }
 
    return result;
}

public String stripHtml(String html) {
    // Remove HTML tags using a regular expression
    return html.replaceAll("<[^>]*>", "");
}


// Call validate method with injected variable
return validate(element, formData, values);