Short description

As title says, a right click on a DockLabel followed by moving a mouse with a right button pressed raises

AttributeError: 'DockLabel' object has no attribute 'pressPos'.

It only happens if a DockLabel was not clicked before.

Code to reproduce

from pyqtgraph.dockarea import DockArea, Dock
from pyqtgraph.Qt import QtGui

app = QtGui.QApplication([])
dock_area = DockArea()
dock = Dock('Dock label')
dock_area.addDock(dock)
dock_area.show()
QtGui.QApplication.exec_()

# Then move mouse over a label 'Dock label', press right button and move mouse

Expected behavior

I assume, nothing should happen.

Real behavior

Program crashes with the following:

Traceback (most recent call last):
File "C:...\pyqtgraph\dockarea\Dock.py", line 338, in mouseMoveEvent
if not self.startedDrag and (ev.pos() - self.pressPos).manhattanLength() > QtGui.QApplication.startDragDistance():
AttributeError: 'DockLabel' object has no attribute 'pressPos'

Tested environment(s)

• PyQtGraph version: 0.10.0
• Qt Python binding: PyQt5 5.14.1 Qt 5.14.0
• Python version: 3.7.6
• NumPy version: 1.18.1
• Operating system: Windows 10
• Installation method: pip install