Hi Bright Wing,
The plugin issue is caused by El Capitan not including a workable Tcl implementation as part of its system. Tcl is needed for the python Tk widget gui code to work.
I will look into how to fix it for El Capitan. One workaround would be to install Tcl which python.org recommends you do by installing ActiveState's tcl package for Mac OSX.
As for the segfault, that is a different issue and I will look into the HTMLResource code that invokes that GumboInterface to see if I see any obvious issues exist when it tries to build epub3 manifest properties by scanning the new xhtml file.
Thanks,
KevinH
ps. As for the segfault, I can reproduce this on my machine if I load up an epub3 and then try to Insert Blank Document in the BookBrowser. I will create a debug build to see what in earth is happening and get this fixed for the next release.
pps. I just pushed a fix for the segfault to master. It tried to find manifest properties on a blank xhtml file (whose text had not been set yet) and of course barfed as a result.
I will try to track down what version of Tcl that El Capitan ships with to prevent this error for others. Meanwhile the solution is to Install the community version of ActiveState ActiveTcl recommended by
www.python.org for Mac OS X users for Python 3.
Thanks for your bug report and crash log!
KevinH