TaskTransformedParameters derives from Gui::DocumentObserver to handle deletion of view provider when clicking Cancel button

src/Mod/PartDesign/Gui/TaskLinearPatternParameters.cpp

@@ -370,7 +370,6 @@ unsigned TaskLinearPatternParameters::getOccurrences(void) const
     return ui->spinOccurrences->value();
 }
 
-
 TaskLinearPatternParameters::~TaskLinearPatternParameters()
 {
     //hide the parts coordinate system axis for selection
@@ -381,7 +380,8 @@ TaskLinearPatternParameters::~TaskLinearPatternParameters()
             ViewProviderOrigin* vpOrigin;
             vpOrigin = static_cast<ViewProviderOrigin*>(Gui::Application::Instance->getViewProvider(origin));
             vpOrigin->resetTemporaryVisibility();
-        } catch (const Base::Exception &ex) {
+        }
+        catch (const Base::Exception &ex) {
             Base::Console().Error ("%s\n", ex.what () );
         }
     }

src/Mod/PartDesign/Gui/TaskTransformedParameters.cpp

@@ -72,6 +72,12 @@ TaskTransformedParameters::TaskTransformedParameters(ViewProviderTransformed *Tr
       blockUpdate(false)
 {
     selectionMode = none;
+
+    if (TransformedView) {
+        Gui::Document* doc = TransformedView->getDocument();
+        this->attachDocument(doc);
+        this->enableNotifications(DocumentObserver::Delete);
+    }
 }
 
 TaskTransformedParameters::TaskTransformedParameters(TaskMultiTransformParameters *parentTask)
@@ -92,6 +98,12 @@ TaskTransformedParameters::~TaskTransformedParameters()
     Gui::Selection().rmvSelectionGate();
 }
 
+void TaskTransformedParameters::slotDeletedObject(const Gui::ViewProviderDocumentObject& Obj)
+{
+    if (TransformedView == &Obj)
+        TransformedView = nullptr;
+}
+
 bool TaskTransformedParameters::isViewUpdated() const
 {
     return (blockUpdate == false);
@@ -269,12 +281,13 @@ PartDesign::Transformed *TaskTransformedParameters::getTopTransformedObject() co
     return static_cast<PartDesign::Transformed*>(transform);
 }
 
-
 PartDesign::Transformed *TaskTransformedParameters::getObject() const {
     if (insideMultiTransform)
         return parentTask->getSubFeature();
-    else
+    else if (TransformedView)
         return static_cast<PartDesign::Transformed*>(TransformedView->getObject());
+    else
+        return nullptr;
 }
 
 Part::Feature *TaskTransformedParameters::getBaseObject() const {

src/Mod/PartDesign/Gui/TaskTransformedParameters.h

@@ -30,6 +30,7 @@
 
 #include <Gui/TaskView/TaskView.h>
 #include <Gui/Selection.h>
+#include <Gui/DocumentObserver.h>
 
 #include "TaskFeatureParameters.h"
 #include "TaskTransformedMessages.h"
@@ -114,7 +115,9 @@ private:
   Because in the second case there is no ViewProvider, some special methods are required to
   access the underlying FeatureTransformed object in two different ways.
   **/
-class TaskTransformedParameters : public Gui::TaskView::TaskBox, public Gui::SelectionObserver
+class TaskTransformedParameters : public Gui::TaskView::TaskBox,
+                                  public Gui::SelectionObserver,
+                                  public Gui::DocumentObserver
 {
     Q_OBJECT
 
@@ -184,6 +187,8 @@ protected:
     int getUpdateViewTimeout() const;
 
 protected:
+    /** Notifies when the object is about to be removed. */
+    virtual void slotDeletedObject(const Gui::ViewProviderDocumentObject& Obj);
     virtual void changeEvent(QEvent *e) = 0;
     virtual void onSelectionChanged(const Gui::SelectionChanges& msg) = 0;
     virtual void clearButtons()=0;