Short description

Found a small bug in SignalProxy.__init__() that occurs when creating an instance of SignalProxy without directly handing over a slot to connect to.
The default value for the optional init parameter slot is None and so you can not create a weakref from that.
The culprit is line 38 of SignalProxy.py https://github.com/pyqtgraph/pyqtgraph/blob/develop/pyqtgraph/SignalProxy.py#L38

My proposal would be to get rid of the instance attribute self.slot entirely since you only use it for disconnection in SignalProxy.disconnect. This is not needed as you could simply call self.sigDelayed.disconnect() without any arguments.
This would be even better since it would disconnect ALL slots (even the ones "manually" connected) from sigDelayed as the user would expect from calling disconnect on any QObject (see also https://doc.qt.io/qt-5/qobject.html#disconnect).

Code to reproduce

# -*- coding: utf-8 -*-
import pyqtgraph as pg
import numpy as np

class Dummy(pg.QtCore.QObject):
    sigTest = pg.QtCore.Signal()
    
    def my_slot():
        pass


if __name__ == '__main__':
    app = pg.QtCore.QCoreApplication([])
    dummy_obj = Dummy()
    proxy = pg.SignalProxy(dummy_obj.sigTest)  # raises TypeError
    proxy.sigDelayed.connect(dummy_obj.my_slot)
    # This would work instead:
    # proxy = pg.SignalProxy(dummy_obj.sigTest, slot=dummy_obj.my_slot)

Expected behavior

One should be able to create instances of SignalProxy by just passing the sender signal.
It seems like this was intended behaviour, as it should be, since it can be beneficial to create a proxy and connect the receiving slot later.

Real behavior

Traceback (most recent call last):
  File ".\pg_signal_proxy_test.py", line 15, in <module>
    proxy = pg.SignalProxy(dummy_obj.sigTest)  # raises TypeError
  File "C:\Program Files\Python37\lib\site-packages\pyqtgraph\SignalProxy.py", line 38, in __init__
    self.slot = weakref.ref(slot)
TypeError: cannot create weak reference to 'NoneType' object

Tested environment(s)

• PyQtGraph version: 0.10.0.4
• Qt Python binding: PyQt5 5.9.2 Qt 5.9.3
• Python version: 3.7.7
• NumPy version: 1.18.3
• Operating system: Debian 10 (stable) & Win10 Enterprise (10.0.18363)
• Installation method: pip