Remove the background color and the message string from the interface.xml file
Delete this part
<property key="backgroundColor" type="attribute"> <caption>Color attribute</caption> <category>Appearance</category> <description>The background color of the widget.</description> <attributeTypes> <attributeType name="String" /> </attributeTypes> </property>
If you are interested in building widgets, please also have a look at the new Mendix 8 Pluggable framework. Though the API is a is not fully completed, and does not offer the same capabilities as the dojo widget client API, but you can take leverage of modern TypeScript and React.