Event bug with port connection and navigation

[Nevolutionize]

I've encountered a reproducible bug that occurs with the following actions:

1. Left-click and hold on any port, to draw the pipe
2. Simultaneously, hold the middle-click and pan the graph

This will result in an error in NodeViewer.sceneMousePressEvent, as it cancels out the left click event:

# Traceback (most recent call last):
#   File "...\NodeGraphQt\widgets\scene.py", line 76, in mousePressEvent
#     self.viewer().sceneMousePressEvent(event)
#   File "...\NodeGraphQt\widgets\viewer.py", line 334, in sceneMousePressEvent
#     from_port = pipe.port_from_pos(pos, True)
#   File "...\NodeGraphQt\qgraphics\pipe.py", line 193, in port_from_pos
#     inport_pos = self.input_port.scenePos()
# AttributeError: 'NoneType' object has no attribute 'scenePos'

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Here's a more destructive version of the bug:

1. Left-click and hold on any port, to draw the pipe
2. Move cursor over to any another port
3. While simultaneously holding down left-click, hold the middle-click and pan the graph
4. Move the nodes around; you'll see the pipe floating around in the scene on its own. Selecting it results in the same error we see above and it cannot be removed from that graph instance.

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You've done some amazing work on this nodegraph framework so far. Thanks for making it available!