[Lazarus] Error: unit not found: Classes

[Kjow]

Lazarus: r28973M
FPC 2.4.2: r16754

Double clicking on any component (Form, Button, etc) to create the
relative event or pressing ctrl+space on the source editor, causes
this error:

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C:\Develop\lazarus\lcl\lclclasses.pp(32,10) Error: unit not found: Classes
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