silx.gui.plot.tools.profile: Fixed profile error when plot item is None #1726

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5	10m 10s
Python 3.8 on ubuntu-20.04 PyQt5	10m 10s
Python 3.11 on ubuntu-latest PyQt6	10m 13s
Python 3.11 on ubuntu-latest PyQt6	10m 13s
Python 3.12 on ubuntu-latest PySide6	9m 15s
Python 3.12 on ubuntu-latest PySide6	9m 15s
Python 3.10 on macos-13 PyQt5	11m 10s
Python 3.10 on macos-13 PyQt5	11m 10s
Python 3.12 on macos-13 PyQt6	13m 31s
Python 3.12 on macos-13 PyQt6	13m 31s
Python 3.9 on macos-13 PySide6	10m 51s
Python 3.9 on macos-13 PySide6	10m 2s
Python 3.9 on windows-latest PyQt5	11m 43s
Python 3.9 on windows-latest PyQt5	11m 43s
Python 3.12 on windows-latest PyQt6	11m 40s
Python 3.12 on windows-latest PyQt6	11m 40s
Python 3.10 on windows-latest PySide6	10m 29s
Python 3.10 on windows-latest PySide6	10m 29s
	3h 17m 15s

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