IntelliJ IDEA's Src Directory Gotcha

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Myself and a mate where trying to fix why he couldn’t assign his newly created Application mxml component as the module Main Class thus preventing him from running a build. Simple oversight I noticed is that somehow the src folder had become de-selected as a Sources directory. This is easily fixed by going to the projects Module Settings > Modules > Sources and then select the src directory and then select the Sources tab at the top right of the panel: