v2.0-rc1 not starting, runtime not building

Hi,

I just did a pull from Git and trouble starting pgAdmin4 since.
I am now at commit 512afd66182f463e6ce9515910205dcf0221e58c
This is on ubuntu 16.04.03, using python3 and firefox 55.0.2
I did a fresh pip install -r requirements.txt

With Debug = True, i got the following errors and execution aborted:

(pgadmin4) wbloos(at)wb:~/python-files/pgadmin4/web$ python pgAdmin4.py
ERROR: [Errno 2] No such file or directory: 'run'
2017-09-11 17:18:39,412: ERROR pgadmin: Unable to generate javascript
2017-09-11 17:18:39,412: ERROR pgadmin: To run the app ensure that
yarn install command runs successfully
Traceback (most recent call last):
File "pgAdmin4.py", line 79, in <module>
raise Exception("No generated javascript, aborting")
Exception: No generated javascript, aborting

With Debug = False, the server starts (Starting pgAdmin 4. Please navigate
to http://127.0.0.1:5050 in your browser.)
But when visiting that page, there's no CSS is the background and a grey
layer in the foreground, displayiong the text "Loading pgAdmin 4
v2.0-rc1.." without any progress.

I checked where the Erno 2 comes from, it's in the function
try_building_js() of javascript_bundler.py . The line that calls yarn.

I tried entering the web directory and running yarn install, and that's
where i find the same error again:
ERROR: [Errno 2] No such file or directory: 'install'
The same goes for "yarn run" and "yarn --run=bundle:dev"

Is there a file missing?
I'm not sure if this is also the cause of the web page not loading. The
line of code is only executed if Debug=True

Also, the desktop runtime is (still) not building (same in both checked out
versions).

(pgadmin4) wbloos(at)wb:~/python-files/pgadmin4/runtime$ qmake
Project MESSAGE: Building for QT5+...
Project MESSAGE: Using QWebEngine...
Project MESSAGE: Building for Linux/Mac...
Project MESSAGE: Using /home/wbloos/.virtualenvs/pgadmin4/bin/python-config
Project MESSAGE: Python3 detected.
Project ERROR: Unknown module(s) in QT: webenginewidgets

Even though i do have libqt5webkit5-dev installed.
Is there any other diagnosis that i can perform for more insight?

Cheers,
--
Willy-Bas Loos