This problem is not Native-specific.
In Stage 2 of the Twitter tutorial, the "Model The Get Top Trends Service" step, bullet 2 says: "From the Basic section of the palette drag and drop Action (or use the keyboard shortcut 'a'). Name the Action Download URL. Add to the action a trigger (keyboard shortcut 't') and an exit (keyboard shortcut 'e').". It seems that instead of following the instructions I've marked in bold, you've right-clicked the Download URL actio, and selected Add Element -> Control which adds a predefined trigger named Control and with a Nothing data type which is incompatible with the Text type expected by Read Resource's <URL> trigger.
To overcome this:
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