Using Kubuntu 17.04 64bit, just installed Kdenlive 17.04.3 using the custom PPA suggested on the Kdenlive website. After the first execution of the app, I noticed there was no Menubar. I solved the problem by pressing Ctrl+M twice. First time, despite having hidden menubar, I got a warning message about hiding the menubar! By pressing the key combination again, I got the menubar back.
Thanks for the report. Will close this as it seems a config file from your system. I you notice it happening again please let us know.