Gui: Handle exception in showValidExpression (#26721)

* Gui: Handle exception in showValidExpression

If an exception is raised inside showValidExpression() because the
expression cannot be evaluated then handle the exception and show it as
invalid expression.

This fixes issue: https://github.com/FreeCAD/FreeCAD/issues/26501
---------

Co-authored-by: wwmayer <wmayer@freecad.org>

src/Gui/SpinBox.cpp

@@ -91,9 +91,20 @@ void ExpressionSpinBox::showInvalidExpression(const QString& tip)
     p.setColor(QPalette::Active, QPalette::Text, Qt::red);
     lineedit->setPalette(p);
     iconLabel->setToolTip(tip);
+    iconLabel->setPixmap(getIcon(":/icons/button_invalid.svg", QSize(iconHeight, iconHeight)));
 }
 
 void ExpressionSpinBox::showValidExpression(ExpressionSpinBox::Number number)
+{
+    try {
+        showExpression(number);
+    }
+    catch (const Base::Exception& e) {
+        showInvalidExpression(QString::fromUtf8(e.what()));
+    }
+}
+
+void ExpressionSpinBox::showExpression(Number number)
 {
     std::unique_ptr<Expression> result(getExpression()->eval());
     auto* value = freecad_cast<NumberExpression*>(result.get());

src/Gui/SpinBox.h

@@ -73,6 +73,9 @@ protected:
 
     void drawControl(QStyleOptionSpinBox&);
 
+private:
+    void showExpression(Number number);
+
 protected:
     QLineEdit* lineedit;
     QAbstractSpinBox* spinbox;