If an editor is opened, the semantic highlighter is triggered twice:

  1) From ClangEditorDocumentProcessor::onParserFinished
  2) From CppEditorDocument::applyFontSettings

Caller 1) is expected, it creates an m_unit. However, if threads are
scheduled differently, the semantic highlighter triggered from 2) will
encounter an invalid m_unit.

I'm not sure whether the call to 2) is correct at all.

Change-Id: Ia6ff2852e7d38418ebb01edc3eca3680fac1820a
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>