Re #27587 Make suggested changes

-Change behaviour of double click so error does not appear if clicking on the category
-SelectFunctionDialog changed to be modal

qt/widgets/common/inc/MantidQtWidgets/Common/SelectFunctionDialog.h

@@ -13,6 +13,7 @@
 #include "DllOption.h"
 
 #include <QDialog>
+#include <QTreeWidgetItem>
 #include <map>
 
 namespace Ui {
@@ -53,6 +54,7 @@ private:
 
 private slots:
   void searchBoxChanged(const QString &text);
+  void functionDoubleClicked(QTreeWidgetItem *item);
   void acceptFunction();
   void rejectFunction();
 };

qt/widgets/common/src/SelectFunctionDialog.cpp

@@ -21,6 +21,7 @@
 #include <QCompleter>
 #include <QGridLayout>
 #include <QGroupBox>
+#include <QIcon>
 #include <QLabel>
 #include <QPushButton>
 #include <QVBoxLayout>
@@ -43,6 +44,8 @@ SelectFunctionDialog::SelectFunctionDialog(QWidget *parent)
 SelectFunctionDialog::SelectFunctionDialog(
     QWidget *parent, const std::vector<std::string> &restrictions)
     : QDialog(parent), m_form(new Ui::SelectFunctionDialog) {
+  setModal(true);
+  setWindowIcon(QIcon(":/images/MantidIcon.ico"));
   m_form->setupUi(this);
   m_form->errorMessage->hide();
 
@@ -76,7 +79,7 @@ SelectFunctionDialog::SelectFunctionDialog(
   constructFunctionTree(categories, restrictions);
 
   connect(m_form->fitTree, SIGNAL(itemDoubleClicked(QTreeWidgetItem *, int)),
-          this, SLOT(acceptFunction()));
+          this, SLOT(functionDoubleClicked(QTreeWidgetItem *)));
   m_form->fitTree->setToolTip("Select a function type and press OK.");
 
   connect(m_form->buttonBox, SIGNAL(accepted()), this, SLOT(acceptFunction()));
@@ -202,6 +205,11 @@ void SelectFunctionDialog::searchBoxChanged(const QString &text) {
     m_form->searchBox->setCurrentIndex(index);
 }
 
+void SelectFunctionDialog::functionDoubleClicked(QTreeWidgetItem *item) {
+  if (item->childCount() == 0)
+    acceptFunction();
+}
+
 void SelectFunctionDialog::acceptFunction() {
   const auto func = getFunction();
   if (func.isEmpty()) {