Bug 1576165 - Prevent react warning in inline preview r=jlast

Differential Revision: https://phabricator.services.mozilla.com/D43647

--HG--
extra : moz-landing-system : lando

devtools/client/debugger/src/components/Editor/InlinePreviews.js

@@ -54,6 +54,7 @@ class InlinePreviews extends Component<Props> {
           return (
             <InlinePreviewRow
               editor={editor}
+              key={line}
               line={lineNum}
               previews={previews[line]}
               numColumnBreakpoints={numColumnBreakpoints}