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This is the error. I am using the nightly build of QGIS 3.4.2 (code revision 1abbcb4, date 20181202)

An error has occurred while executing Python code:

AttributeError: 'str' object has no attribute 'get'
Traceback (most recent call last):
File "/usr/share/qgis/python/plugins/processing/gui/ProcessingToolbox.py", line 247, in executeAlgorithm
dlg = AlgorithmDialog(alg, self.in_place_mode, iface.mainWindow())
File "/usr/share/qgis/python/plugins/processing/gui/AlgorithmDialog.py", line 80, in init
self.setMainWidget(self.getParametersPanel(alg, self))
File "/usr/share/qgis/python/plugins/processing/gui/AlgorithmDialog.py", line 96, in getParametersPanel
return ParametersPanel(parent, alg, self.in_place)
File "/usr/share/qgis/python/plugins/processing/gui/ParametersPanel.py", line 103, in init
self.initWidgets()
File "/usr/share/qgis/python/plugins/processing/gui/ParametersPanel.py", line 140, in initWidgets
wrapper = WidgetWrapperFactory.create_wrapper(param, self.parent)
File "/usr/share/qgis/python/plugins/processing/gui/wrappers.py", line 1776, in create_wrapper
if param.metadata().get('widget_wrapper', {}).get('class', None) is not None:
AttributeError: 'str' object has no attribute 'get'

Python version: 3.6.7 (default, Oct 22 2018, 11:32:17) [GCC 8.2.0]
QGIS version: 3.4.2-Madeira Madeira, 1abbcb4

Python Path:
/usr/share/qgis/python
/home/fd/.local/share/QGIS/QGIS3/profiles/default/python
/home/fd/.local/share/QGIS/QGIS3/profiles/default/python/plugins
/usr/share/qgis/python/plugins
/usr/lib/python36.zip
/usr/lib/python3.6
/usr/lib/python3.6/lib-dynload
/usr/local/lib/python3.6/dist-packages
/usr/lib/python3/dist-packages
/home/fd/.local/share/QGIS/QGIS3/profiles/default/python