Fix right-click segfault in new workbench

The error appears to be solved in newer versions of sip / python-qt5,
but that isn't what is available on rhel7. This removes the ability to
have a different `MantidDisplayBase` from the workbench, but it doesn't
appear to be a necessary abstraction for now.

qt/applications/workbench/workbench/plugins/workspacewidget.py

@@ -22,7 +22,6 @@ import functools
 # third-party library imports
 from mantid.api import AnalysisDataService, MatrixWorkspace, WorkspaceGroup
 from mantid.kernel import Logger
-from mantidqt.widgets.workspacewidget.mantidtreemodel import MantidTreeModel
 from mantidqt.widgets.workspacewidget.plotselectiondialog import get_plot_selection
 from mantidqt.widgets.workspacewidget.workspacetreewidget import WorkspaceTreeWidget
 from qtpy.QtWidgets import QVBoxLayout
@@ -72,18 +71,17 @@ class WorkspaceWidget(PluginWidget):
         super(WorkspaceWidget, self).__init__(parent)
 
         # layout
-        workspacewidget = WorkspaceTreeWidget(MantidTreeModel())
-        self.workspacewidget = workspacewidget
+        self.workspacewidget = WorkspaceTreeWidget()
         layout = QVBoxLayout()
         layout.addWidget(self.workspacewidget)
         self.setLayout(layout)
 
         # behaviour
-        workspacewidget.plotSpectrumClicked.connect(functools.partial(self._do_plot_spectrum,
-                                                                      errors=False))
-        workspacewidget.plotSpectrumWithErrorsClicked.connect(functools.partial(self._do_plot_spectrum,
-                                                                                errors=True))
-        workspacewidget.plotColorfillClicked.connect(self._do_plot_colorfill)
+        self.workspacewidget.plotSpectrumClicked.connect(functools.partial(self._do_plot_spectrum,
+                                                                           errors=False))
+        self.workspacewidget.plotSpectrumWithErrorsClicked.connect(functools.partial(self._do_plot_spectrum,
+                                                                                     errors=True))
+        self.workspacewidget.plotColorfillClicked.connect(self._do_plot_colorfill)
 
     # ----------------- Plugin API --------------------
 

qt/python/mantidqt/widgets/src/_widgetscore.sip

@@ -214,8 +214,7 @@ class WorkspaceTreeWidgetSimple : WorkspaceTreeWidget {
 #include "MantidQtWidgets/Common/WorkspacePresenter/WorkspaceTreeWidgetSimple.h"
 %End
 public:
-  WorkspaceTreeWidgetSimple(MantidDisplayBase *mdb /Transfer/,
-                            QWidget *parent /TransferThis/ = nullptr);
+  WorkspaceTreeWidgetSimple(QWidget *parent /TransferThis/ = nullptr);
 
 signals:
   void plotSpectrumClicked(const QStringList & workspaceName);

qt/widgets/common/inc/MantidQtWidgets/Common/WorkspacePresenter/WorkspaceTreeWidgetSimple.h

@@ -50,9 +50,7 @@ class EXPORT_OPT_MANTIDQT_COMMON WorkspaceTreeWidgetSimple
     : public WorkspaceTreeWidget {
   Q_OBJECT
 public:
-  explicit WorkspaceTreeWidgetSimple(
-      MantidQt::MantidWidgets::MantidDisplayBase *mdb,
-      QWidget *parent = nullptr);
+  explicit WorkspaceTreeWidgetSimple(QWidget *parent = nullptr);
   ~WorkspaceTreeWidgetSimple();
 
   // Context Menu Handlers

qt/widgets/common/src/WorkspacePresenter/WorkspaceTreeWidgetSimple.cpp

 #include "MantidQtWidgets/Common/WorkspacePresenter/WorkspaceTreeWidgetSimple.h"
 #include <MantidQtWidgets/Common/MantidTreeWidget.h>
 #include <MantidQtWidgets/Common/MantidTreeWidgetItem.h>
+#include "MantidQtWidgets/Common/MantidTreeModel.h"
 
 #include <MantidAPI/AlgorithmManager.h>
 #include <MantidAPI/FileProperty.h>
@@ -17,9 +18,8 @@ using namespace Mantid::Kernel;
 namespace MantidQt {
 namespace MantidWidgets {
 
-WorkspaceTreeWidgetSimple::WorkspaceTreeWidgetSimple(MantidDisplayBase *mdb,
-                                                     QWidget *parent)
-    : WorkspaceTreeWidget(mdb, parent),
+WorkspaceTreeWidgetSimple::WorkspaceTreeWidgetSimple(QWidget *parent)
+    : WorkspaceTreeWidget(new MantidTreeModel(), parent),
       m_plotSpectrum(new QAction("spectrum...", this)),
       m_plotSpectrumWithErrs(new QAction("spectrum with errors...", this)),
       m_plotColorfill(new QAction("colorfill", this)) {