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>
Dear Omkar,
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.
https://docs.mendix.com/howto/extensibility/pluggable-widgets
Cheers, Andries