Added an additional guard clause to fix the bug that causes an IndexError while interacting with the plots in the FitPropertyBrowser widget (#40574)

### Description of work

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Closes #40426. <!-- One line per closed issue. -->

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### To test:
The error was reported by a user through the error reporter. It is hard
to reproduce but the stack trace clearly identifies the line to fix.


Co-authored-by: Waruna Priyankara J A Wickramasingha <[email protected]>

Author: RabiyaF

docs/source/release/v6.15.0/Workbench/Bugfixes/40426.rst

@@ -0,0 +1 @@
+- Added an additional guard clause to fix the bug that causes an IndexError while interacting with the plots in the ``FitPropertyBrowser`` widget.

qt/python/mantidqt/mantidqt/widgets/fitpropertybrowser/fitpropertybrowserplotinteraction.py

@@ -300,7 +300,7 @@ def _shift_stored_prefixes(self, removed_prefix):
         :param removed_prefix: Prefix of the function removed e.g f0
         :return:
         """
-        if not self.guess_lines or removed_prefix[1] == len(self.guess_lines):
+        if not self.guess_lines or (len(removed_prefix) < 2) or (removed_prefix[1] == len(self.guess_lines)):
             return
         for prefixed_function in reversed(list(self.guess_lines)):
             prefix_and_function = prefixed_function.split(".")

qt/python/mantidqt/mantidqt/widgets/test/test_fitpropertybrowserplotinteraction.py

@@ -170,6 +170,14 @@ def test_remove_function_correctly_updates_stored_prefixed_functions(self):
 
         self.assertEqual(list(self.browser_plot_interaction.guess_lines.keys()), ["f0.FlatBackground", "f1.GausOsc"])
 
+    def test_shift_stored_prefixes_returns_when_prefix_len_is_less_than_two(self):
+        self.create_mock_guess_lines()
+        self.setup_mock_fit_browser(
+            workspace_creator=self.create_workspace2D, workspace_name="test_workspace", function=FULL_FUNCTION, function_prefix="f"
+        )
+        self.browser_plot_interaction.slot_for_function_removed()
+        self.assertEqual(list(self.browser_plot_interaction.guess_lines.keys()), ["f0.FlatBackground", "f1.LinearBackground", "f2.GausOsc"])
+
     def test_remove_function_correctly_removes_line(self):
         workspace_name = "test_workspace"
         prefix = "f1"