From the changelog for 1.3.6.- 

Is this the

 Assertion failure in -[NSSavePanel _attachSandboxExtensions:toURL:orURLs:], 

error ?

I am using 1.4 and still get this error.

>>>

Fl_Native_File_Chooser native;

char title[FILENAME_MAX];

char* filename_name, * filename_ext;

native.options(Fl_Native_File_Chooser::NEW_FOLDER |  Fl_Native_File_Chooser::SAVEAS_CONFIRM | Fl_Native_File_Chooser::USE_FILTER_EXT);

        strcpy(title, "Select output file");

        native.title(title);

switch (native.show()) {.  <<------ CRASH

}