[MBsysPy][BugFix][MbsDirdyn] Getting option resfilename was crashing if no...

[MBsysPy][BugFix][MbsDirdyn] Getting option resfilename was crashing if no name was specified by user.

MBsysC/mbs_interface/MBsysPy/mbsyspy/mbs_dirdyn.py

@@ -696,7 +696,11 @@ class MbsDirdyn(object):
             elif key == "save2file":
                 options.append(self.mbs_dirdyn_ptr.contents.options.contents.save2file)
             elif key == "resfilename":
-                options.append(bytes_to_str(ctypes.string_at(self.mbs_dirdyn_ptr.contents.options.contents.resfilename)))
+                ptr = self.mbs_dirdyn_ptr.contents.options.contents.resfilename
+                if ptr is not None:
+                    options.append(bytes_to_str(ctypes.string_at(self.mbs_dirdyn_ptr.contents.options.contents.resfilename)))
+                else:
+                    options.append("dirdyn")
             elif key == "respath":
                 options.append(bytes_to_str(ctypes.string_at(self.mbs_dirdyn_ptr.contents.options.contents.respath)))
             elif key == "animpath":