If you try to install Nokia QtSDK on Ubuntu using the Nokia installer (that provides a newer version than the one distributed in Ubuntu Software Center) you could get an error like this:

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(Qt_SDK_Lin32_offline_v1_1_3_en.run:3126): Gtk-CRITICAL **:
IA__gtk_widget_style_get: assertion `GTK_IS_WIDGET (widget)`
failed  

to fix it, you need to run the installer with a specific parameter:

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./Qt_SDK_Lin32_offline_v1_1_4_en.run -style cleanlooks  

and everything should work!