QWidget: Must construct a QApplication before a QWidget

There are instances that you download official examples from http://www.vtk.org/Wiki/VTK/Examples/Cxx/Qt. but got error like "QWidget: Must construct a QApplication before a QWidget" during runtime. It is most probably due to the debug and release conflict between Qt libaray and your exe file. Just rebuild your exe file by set CMAKE_BUILD_TYPE to Debug or Release correspondingly. It can be done by modifying CMakefile.txt as
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set(CMAKE_BUILD_TYPE Debug) or set(CMAKE_BUILD_TYPE Release)
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or running cmake like
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cmake -DCMAKE_BUILD_TYPE=Release
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