Merge pull request #37074 from robertapplin/37071-fix-crash-due-to-tw…

…ice-observer-subscribing

Fix crash due to subscribing observer twice to notification center

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
 

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);
 }
 
 /**