From 8a1c6d3352dca695128396348866433613b2f5d4 Mon Sep 17 00:00:00 2001
From: Applin <robert.applin@stfc.ac.uk>
Date: Mon, 25 Mar 2024 13:45:42 +0000
Subject: [PATCH 1/2] Ensure a poco observer is only subscribed once to the
 Notification center

---
 qt/widgets/plotting/src/PreviewPlot.cpp | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/qt/widgets/plotting/src/PreviewPlot.cpp b/qt/widgets/plotting/src/PreviewPlot.cpp
index 99d54d5bb9e6..230e7d240662 100644
--- a/qt/widgets/plotting/src/PreviewPlot.cpp
+++ b/qt/widgets/plotting/src/PreviewPlot.cpp
@@ -43,6 +43,16 @@ constexpr auto LOG_SCALE = "Log";
 constexpr auto SQUARE_SCALE = "Square";
 constexpr auto SHOWALLERRORS = "Show all errors";
 constexpr auto HIDEALLERRORS = "Hide all errors";
+
+template <typename Observer> void modifyObserver(Observer &observer, bool const turnOn) {
+  auto &notificationCenter = AnalysisDataService::Instance().notificationCenter;
+  if (turnOn && !notificationCenter.hasObserver(observer)) {
+    notificationCenter.addObserver(observer);
+  } else if (!turnOn && notificationCenter.hasObserver(observer)) {
+    notificationCenter.removeObserver(observer);
+  }
+}
+
 } // namespace
 
 namespace MantidQt::MantidWidgets {
@@ -77,14 +87,8 @@ PreviewPlot::~PreviewPlot() { watchADS(false); }
  * @param on If true ADS observers are enabled else they are disabled
  */
 void PreviewPlot::watchADS(bool on) {
-  auto &notificationCenter = AnalysisDataService::Instance().notificationCenter;
-  if (on) {
-    notificationCenter.addObserver(m_wsRemovedObserver);
-    notificationCenter.addObserver(m_wsReplacedObserver);
-  } else {
-    notificationCenter.removeObserver(m_wsReplacedObserver);
-    notificationCenter.removeObserver(m_wsRemovedObserver);
-  }
+  modifyObserver(m_wsReplacedObserver, on);
+  modifyObserver(m_wsRemovedObserver, on);
 }
 
 /**

From 033be36cf53647efa3f7b0749acdbc542ea606b3 Mon Sep 17 00:00:00 2001
From: Applin <robert.applin@stfc.ac.uk>
Date: Mon, 25 Mar 2024 13:50:19 +0000
Subject: [PATCH 2/2] Add release note

---
 docs/source/release/v6.9.1/index.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/docs/source/release/v6.9.1/index.rst b/docs/source/release/v6.9.1/index.rst
index bee08622220a..e0fc0a8c8171 100644
--- a/docs/source/release/v6.9.1/index.rst
+++ b/docs/source/release/v6.9.1/index.rst
@@ -15,6 +15,7 @@ The changes are:
  - Removed slit lookup that was specific to OFFSPEC in :ref:`algm-ReflectometryReductionOneLiveData` as it is no longer required and was causing regular crashes when running live data on OFFSPEC.
  - Fixed a bug in :ref:`Elwin Tab <elwin>` of :ref:`Data Manipulation Interface <interface-inelastic-data-manipulation>` where changing integration range with the sliders did not change default integration range.
  - Add sample log values to the live data workspace before the instrument is loaded in :ref:`algm-ReflectometryReductionOneLiveData` to ensure log values are available when setting the detector positions.
+ - Fixed a crash when using multiple Indirect or Inelastic interfaces. This crash was present on the :ref:`Bayes Fitting <interface-inelastic-bayes-fitting>` interface, but could also be replicated elsewhere.
 
 Citation
 --------
@@ -36,6 +37,7 @@ Changes in this version
 * `37053 <https://github.com/mantidproject/mantid/pull/37053>`_ Load instrument after loading sample logs in ReflectometryReductionOneLiveData
 * `37016 <https://github.com/mantidproject/mantid/pull/37016>`_ Fix sliders not changing integration limits in Elwin tab
 * `36935 <https://github.com/mantidproject/mantid/pull/36935>`_ Fix regular live data crashes on OFFSPEC
+* `37074 <https://github.com/mantidproject/mantid/pull/37074>`_ Fix crash on Indirect/Inelastic interfaces.
 
 .. _download page: http://download.mantidproject.org