Prevent crash when all entries are deleted from a group

* Fix #4093 - The first entry in the list is selected after deleting an entry
* Prevents crashes due to dangling pointers held by the Entry Preview Widget when entries were deleted.
* Improve GUI tests to ensure this new behavior occurs.

src/gui/DatabaseWidget.cpp

@@ -154,6 +154,7 @@ DatabaseWidget::DatabaseWidget(QSharedPointer<Database> db, QWidget* parent)
     m_shareLabel->setVisible(false);
 #endif
 
+    m_previewView->setObjectName("previewWidget");
     m_previewView->hide();
     m_previewSplitter->addWidget(m_entryView);
     m_previewSplitter->addWidget(m_previewView);
@@ -552,6 +553,14 @@ void DatabaseWidget::deleteEntries(QList<Entry*> selectedEntries)
     }
 
     refreshSearch();
+
+    m_entryView->setFirstEntryActive();
+    auto* currentEntry = currentSelectedEntry();
+    if (currentEntry) {
+        m_previewView->setEntry(currentEntry);
+    } else {
+        m_previewView->setGroup(groupView()->currentGroup());
+    }
 }
 
 bool DatabaseWidget::confirmDeleteEntries(QList<Entry*> entries, bool permanent)

src/gui/EntryPreviewWidget.cpp

@@ -145,10 +145,12 @@ void EntryPreviewWidget::setDatabaseMode(DatabaseWidget::Mode mode)
     }
 
     if (mode == DatabaseWidget::Mode::ViewMode) {
-        if (m_ui->stackedWidget->currentWidget() == m_ui->pageGroup) {
+        if (m_currentGroup && m_ui->stackedWidget->currentWidget() == m_ui->pageGroup) {
             setGroup(m_currentGroup);
-        } else {
+        } else if (m_currentEntry) {
             setEntry(m_currentEntry);
+        } else {
+            hide();
         }
     }
 }

src/gui/EntryPreviewWidget.h

@@ -77,8 +77,8 @@ private:
 
     const QScopedPointer<Ui::EntryPreviewWidget> m_ui;
     bool m_locked;
-    Entry* m_currentEntry;
-    Group* m_currentGroup;
+    QPointer<Entry> m_currentEntry;
+    QPointer<Group> m_currentGroup;
     QTimer m_totpTimer;
     quint8 m_selectedTabEntry;
     quint8 m_selectedTabGroup;