[JimmXinu]

Just in case you are interested, here's a modified version of launch_app(). I've only tested it with windows.

Spoiler:

    def launch_app(self, external_app_path, app_args, path_to_file=None):
        external_app_path = os.path.expandvars(external_app_path)
        if path_to_file != None:
            path_to_file = path_to_file.encode('utf-8')
        if DEBUG:
            print('Open: ', external_app_path, '(file): ', path_to_file, ' (args): ', app_args)

        if isosx:
            # For OSX we will not support optional command line arguments currently
            if external_app_path.lower().endswith(".app"):
                args = 'open -a '
            else:
                args = ''
            args += '"%s"' % external_app_path
            if path_to_file != None:
                args += ' "%s"' % path_to_file
            subprocess.Popen(args, shell=True)

        else:
            # For Windows/Linux merge any optional command line args with the app/file paths
            app_args_list = []
            if app_args:
                app_args_list = app_args.split(',')
            app_args_list.insert(0, external_app_path)
            if path_to_file != None:
                app_args_list.append(path_to_file)
            if iswindows:
                if path_to_file != None:
                    # Add to the recently opened files list to support windows jump lists etc.
                    from win32com.shell import shell, shellcon
                    shell.SHAddToRecentDocs(shellcon.SHARD_PATHA, path_to_file)
                # As of v1.5.3 will no longer use subprocess because it does not work
                # for users who have non-ascii library paths
                # However we need a special case for Sigil which has issues with C runtime paths
                DETACHED_PROCESS = 0x00000008
                if external_app_path.lower().endswith('sigil.exe'):
                    clean_env = dict(os.environ)
                    del clean_env['PATH']
                    subprocess.Popen(app_args_list, creationflags=DETACHED_PROCESS, env=clean_env)
                else:
                    from win32process import CreateProcess, STARTUPINFO
                    import _subprocess
                    cmd_line = '"%s"'%app_args_list[0]
                    for app_arg in app_args_list[1:]:
                        cmd_line += ' "%s"'%app_arg
                    si = STARTUPINFO()
                    si.dwFlags |= _subprocess.STARTF_USESTDHANDLES
                    print("OW cmd_line:%s"%cmd_line)
                    CreateProcess(None, cmd_line, None, None, False, DETACHED_PROCESS, None, None, si)

            else: #Linux
                clean_env = dict(os.environ)
                clean_env['LD_LIBRARY_PATH'] = ''
                subprocess.Popen(app_args_list, env=clean_env)