[PD] fix dressup feature display when broken

As discussed in https://forum.freecadweb.org/viewtopic.php?p=479647#p479377
the user can end up without either seeing nothing or an inexisting dressup feature.

This PR fixes this.

src/Mod/PartDesign/Gui/TaskDraftParameters.cpp

@@ -118,6 +118,9 @@ TaskDraftParameters::TaskDraftParameters(ViewProviderDressUp *DressUpView, QWidg
     ref = pcDraft->PullDirection.getValue();
     strings = pcDraft->PullDirection.getSubValues();
     ui->lineLine->setText(getRefStr(ref, strings));
+
+    // the dialog can be called on a broken draft, then hide the draft
+    hideOnError();
 }
 
 void TaskDraftParameters::onSelectionChanged(const Gui::SelectionChanges& msg)
@@ -171,6 +174,8 @@ void TaskDraftParameters::onSelectionChanged(const Gui::SelectionChanges& msg)
             pcDraft->getDocument()->recomputeFeature(pcDraft);
             // highlight existing references for possible further selections
             DressUpView->highlightReferences(true);
+            // hide the draft if there was a computation error
+            hideOnError();
         } else if (selectionMode == line) {
             PartDesign::Draft* pcDraft = static_cast<PartDesign::Draft*>(DressUpView->getObject());
             std::vector<std::string> edges;
@@ -185,6 +190,8 @@ void TaskDraftParameters::onSelectionChanged(const Gui::SelectionChanges& msg)
             pcDraft->getDocument()->recomputeFeature(pcDraft);
             // highlight existing references for possible further selections
             DressUpView->highlightReferences(true);
+            // hide the draft if there was a computation error
+            hideOnError();
         }
     }
 }
@@ -260,6 +267,8 @@ void TaskDraftParameters::onRefDeleted(void)
     pcDraft->Base.setValue(base, refs);
     // recompute the feature
     pcDraft->recomputeFeature();
+    // hide the draft if there was a computation error
+    hideOnError();
 
     // if there is only one item left, it cannot be deleted
     if (ui->listWidgetReferences->count() == 1) {
@@ -295,6 +304,8 @@ void TaskDraftParameters::onAngleChanged(double angle)
     setupTransaction();
     pcDraft->Angle.setValue(angle);
     pcDraft->getDocument()->recomputeFeature(pcDraft);
+    // hide the draft if there was a computation error
+    hideOnError();
 }
 
 double TaskDraftParameters::getAngle(void) const
@@ -308,6 +319,8 @@ void TaskDraftParameters::onReversedChanged(const bool on) {
     setupTransaction();
     pcDraft->Reversed.setValue(on);
     pcDraft->getDocument()->recomputeFeature(pcDraft);
+    // hide the draft if there was a computation error
+    hideOnError();
 }
 
 bool TaskDraftParameters::getReversed(void) const
@@ -373,8 +386,6 @@ TaskDlgDraftParameters::~TaskDlgDraftParameters()
 
 bool TaskDlgDraftParameters::accept()
 {
-    parameter->showObject();
-
     std::vector<std::string> strings;
     App::DocumentObject* obj;
     TaskDraftParameters* draftparameter = static_cast<TaskDraftParameters*>(parameter);