invalid initialization of reference of type ‘nux::NString&’ from expression of type ‘std::string {aka std::basic_string<char>}’

When I try to build the nuxplayground I get the following error:

make[3]: Entering directory `/home/eleni/staging/source/playgroundbranches/nuxplayground/src/visual-fx'
  CXX AbstractEffectShader.o
AbstractEffectShader.cpp: In constructor ‘AbstractEffectShader::AbstractEffectShader()’:
AbstractEffectShader.cpp:54:80: error: invalid initialization of reference of type ‘nux::NString&’ from expression of type ‘std::string {aka std::basic_string<char>}’
In file included from /home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/NuxCore.h:723:0,
                 from /home/eleni/staging/installs/nux/include/Nux-3.0/Nux/Nux.h:26,
                 from AbstractEffectShader.cpp:1:
/home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/FileIO.h:32:8: error: in passing argument 1 of ‘bool nux::LoadFileToString(nux::NString&, const TCHAR*, nux::NFileManager&)’
AbstractEffectShader.cpp:72:82: error: invalid initialization of reference of type ‘nux::NString&’ from expression of type ‘std::string {aka std::basic_string<char>}’
In file included from /home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/NuxCore.h:723:0,
                 from /home/eleni/staging/installs/nux/include/Nux-3.0/Nux/Nux.h:26,
                 from AbstractEffectShader.cpp:1:
/home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/FileIO.h:32:8: error: in passing argument 1 of ‘bool nux::LoadFileToString(nux::NString&, const TCHAR*, nux::NFileManager&)’
AbstractEffectShader.cpp:91:80: error: invalid initialization of reference of type ‘nux::NString&’ from expression of type ‘std::string {aka std::basic_string<char>}’
In file included from /home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/NuxCore.h:723:0,
                 from /home/eleni/staging/installs/nux/include/Nux-3.0/Nux/Nux.h:26,
                 from AbstractEffectShader.cpp:1:
/home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/FileIO.h:32:8: error: in passing argument 1 of ‘bool nux::LoadFileToString(nux::NString&, const TCHAR*, nux::NFileManager&)’
AbstractEffectShader.cpp:108:77: error: invalid initialization of reference of type ‘nux::NString&’ from expression of type ‘std::string {aka std::basic_string<char>}’
In file included from /home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/NuxCore.h:723:0,
                 from /home/eleni/staging/installs/nux/include/Nux-3.0/Nux/Nux.h:26,
                 from AbstractEffectShader.cpp:1:
/home/eleni/staging/installs/nux/include/Nux-3.0/NuxCore/FileIO.h:32:8: error: in passing argument 1 of ‘bool nux::LoadFileToString(nux::NString&, const TCHAR*, nux::NFileManager&)’
make[3]: *** [AbstractEffectShader.o] Error 1
make[3]: Leaving directory `/home/eleni/staging/source/playgroundbranches/nuxplayground/src/visual-fx'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/home/eleni/staging/source/playgroundbranches/nuxplayground/src'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/eleni/staging/source/playgroundbranches/nuxplayground'
make: *** [all] Error 2