Fix wxGUI Histogramming Tool Plot xSpec, ySpec property value

[tmszi]

Reproduce:

1. In the Layer Manager toolbar click on the Add raster map layer icon tool
2. Right click on the added raster layer to invoke layer menu and click on the Histogram menu item
3. On the Histogramming Tool frame toolbar click on the Settings icon tool menu and choose Plot settings (open dialog)
4. On the Plot settings dialog, change X-Axis Scale option to the custom type, and set Custom min and max value, and change Y-Axis Scale same too
5. On the Plot settings dialog click on the Ok button
6. Close Histogramming Tool frame
7. Reopen Histogramming Tool frame

Error message appear in the Console page:

Traceback (most recent call last):
  File "/usr/local/grass79/gui/wxpython/lmgr/layertree.py",
line 1130, in OnHistogram

win = HistogramPlotFrame(parent=self, rasterList=rasterList)
  File
"/usr/local/grass79/gui/wxpython/wxplot/histogram.py", line
73, in __init__

self._initOpts()
  File
"/usr/local/grass79/gui/wxpython/wxplot/histogram.py", line
85, in _initOpts

self.InitPlotOpts('histogram')
  File "/usr/local/grass79/gui/wxpython/wxplot/base.py",
line 199, in InitPlotOpts

self.client.xSpec =
self.properties['x-axis']['prop']['type']
  File "/usr/lib/python3/dist-
packages/wx/lib/plot/plotcanvas.py", line 1564, in xSpec

if not isinstance(value, (list, tuple)) and len(value != 2):
TypeError
:
object of type 'bool' has no len()