Fix crash on exit when having edited a text area

For some reason Q/KTextEdit is sending textChanged inside the
destructor, so protect us from that.

BUG: 393334

autotests/parttest.cpp

@@ -21,6 +21,7 @@
 
 #include "../generators/poppler/config-okular-poppler.h"
 
+#include <KActionCollection>
 #include <KConfigDialog>
 #include <KParts/OpenUrlArguments>
 
@@ -29,6 +30,7 @@
 #include <QPushButton>
 #include <QScrollBar>
 #include <QTemporaryDir>
+#include <QTextEdit>
 #include <QTreeView>
 #include <QUrl>
 #include <QDesktopServices>
@@ -108,6 +110,7 @@ class PartTest
         void testClickAnywhereAfterSelectionShouldUnselect();
         void testeRectSelectionStartingOnLinks();
         void testCheckBoxReadOnly();
+        void testCrashTextEditDestroy();
 
     private:
         void simulateMouseSelection(double startX, double startY, double endX, double endY, QWidget *target);
@@ -1399,6 +1402,18 @@ void PartTest::testCheckBoxReadOnly()
     QVERIFY( !targetDefaultRO->isReadOnly() );
 }
 
+void PartTest::testCrashTextEditDestroy()
+{
+    const QString testFile = QStringLiteral( KDESRCDIR "data/formSamples.pdf" );
+    Okular::Part part( nullptr, nullptr, QVariantList() );
+    part.openDocument( testFile );
+    part.widget()->show();
+    QVERIFY(QTest::qWaitForWindowExposed(part.widget()));
+
+    part.widget()->findChild<QTextEdit*>()->setText("HOLA");
+    part.actionCollection()->action(QStringLiteral("view_toggle_forms"))->trigger();
+}
+
 }
 
 int main(int argc, char *argv[])

ui/formwidgets.cpp

@@ -292,6 +292,7 @@ FormWidgetIface::FormWidgetIface( QWidget * w, Okular::FormField * ff )
 
 FormWidgetIface::~FormWidgetIface()
 {
+    m_ff = nullptr;
 }
 
 Okular::NormalizedRect FormWidgetIface::rect() const
@@ -668,6 +669,10 @@ void TextAreaEdit::slotHandleTextChangedByUndoRedo( int pageNumber,
 
 void TextAreaEdit::slotChanged()
 {
+    // happens on destruction
+    if (!m_ff)
+        return;
+
     Okular::FormFieldText *form = static_cast<Okular::FormFieldText *>(m_ff);
     QString contents = toPlainText();
     int cursorPos = textCursor().position();