diff --git a/spyder/plugins/editor/panels/scrollflag.py b/spyder/plugins/editor/panels/scrollflag.py
index 5a761f24935..1ae496f039d 100644
--- a/spyder/plugins/editor/panels/scrollflag.py
+++ b/spyder/plugins/editor/panels/scrollflag.py
@@ -20,6 +20,7 @@
 # Local imports
 from spyder.api.panel import Panel
 from spyder.plugins.completion.api import DiagnosticSeverity
+from spyder.plugins.editor.utils.editor import is_block_safe
 
 
 REFRESH_RATE = 1000
@@ -221,7 +222,7 @@ def paintEvent(self, event):
             if editor.verticalScrollBar().maximum() == 0:
                 # No scroll
                 for block in dict_flag_lists[flag_type]:
-                    if not block.isValid():
+                    if not is_block_safe(block):
                         continue
                     geometry = editor.blockBoundingGeometry(block)
                     rect_y = ceil(
@@ -233,7 +234,7 @@ def paintEvent(self, event):
             elif last_line == 0:
                 # Only one line
                 for block in dict_flag_lists[flag_type]:
-                    if not block.isValid():
+                    if not is_block_safe(block):
                         continue
                     rect_y = ceil(first_y_pos)
                     painter.drawRect(rect_x, rect_y, rect_w, rect_h)
@@ -243,7 +244,7 @@ def paintEvent(self, event):
                     # If the file is too long, do not freeze the editor
                     next_line = 0
                     for block in dict_flag_lists[flag_type]:
-                        if not block.isValid():
+                        if not is_block_safe(block):
                             continue
                         block_line = block.firstLineNumber()
                         # block_line = -1 if invalid
diff --git a/spyder/plugins/editor/utils/editor.py b/spyder/plugins/editor/utils/editor.py
index 1b6b127d54d..28096710399 100644
--- a/spyder/plugins/editor/utils/editor.py
+++ b/spyder/plugins/editor/utils/editor.py
@@ -51,6 +51,24 @@ def drift_color(base_color, factor=110):
             return base_color.lighter(factor + 10)
 
 
+def is_block_safe(block):
+    """
+    Check if the block is safe to work with.
+
+    A BlockUserData must have been set on the block while it was known to be
+    safe.
+
+    If an editor is cleared by editor.clear() or editor.set_text() for example,
+    all old blocks will continue to report block.isValid() == True, but will
+    raise a segmentation fault on almost all methods.
+
+    One way to check if a block is valid is that the userData is reset to None
+    or QTextBlockUserData. So if a block is known to have setUserData to
+    BlockUserData, this fact can be used to check the block.
+    """
+    return block.isValid() and isinstance(block.userData(), BlockUserData)
+
+
 class BlockUserData(QTextBlockUserData):
     def __init__(self, editor, color=None, selection_start=None,
                  selection_end=None):
@@ -67,15 +85,6 @@ def __init__(self, editor, color=None, selection_start=None,
         self.selection_start = selection_start
         self.selection_end = selection_end
 
-        # Add a reference to the user data in the editor as the block won't.
-        # The list should /not/ be used to list BlockUserData as the blocks
-        # they refer to might not exist anymore.
-        # This prevents a segmentation fault.
-        if editor is None:
-            # Won't be destroyed
-            self.refloop = self
-            return
-
     def _selection(self):
         """
         Function to compute the selection.
diff --git a/spyder/plugins/editor/widgets/codeeditor.py b/spyder/plugins/editor/widgets/codeeditor.py
index d18c33548ea..73c3b23d486 100644
--- a/spyder/plugins/editor/widgets/codeeditor.py
+++ b/spyder/plugins/editor/widgets/codeeditor.py
@@ -2579,7 +2579,12 @@ def __mark_occurrences(self):
         extra_selections = self.get_extra_selections('occurrences')
         first_occurrence = None
         while cursor:
-            self.occurrences.append(cursor.block())
+            block = cursor.block()
+            if not block.userData():
+                # Add user data to check block validity
+                block.setUserData(BlockUserData(self))
+            self.occurrences.append(block)
+
             selection = self.get_selection(cursor)
             if len(selection.cursor.selectedText()) > 0:
                 extra_selections.append(selection)
@@ -2623,7 +2628,13 @@ def highlight_found_results(self, pattern, word=False, regexp=False,
             selection = TextDecoration(self.textCursor())
             selection.format.setBackground(self.found_results_color)
             selection.cursor.setPosition(pos1)
-            self.found_results.append(selection.cursor.block())
+
+            block = selection.cursor.block()
+            if not block.userData():
+                # Add user data to check block validity
+                block.setUserData(BlockUserData(self))
+            self.found_results.append(block)
+
             selection.cursor.setPosition(pos2, QTextCursor.KeepAnchor)
             extra_selections.append(selection)
         self.set_extra_selections('find', extra_selections)