[SuperEditor] - Fix crash with selected color strategy in an empty paragraph (Resolves #2253) (#2539)

Author: angelosilvestre

super_editor/lib/src/default_editor/layout_single_column/_styler_user_selection.dart

@@ -134,20 +134,22 @@ class SingleColumnLayoutSelectionStyler extends SingleColumnLayoutStylePhase {
       if (viewModel is TextComponentViewModel) {
         final componentTextColor = viewModel.textStyleBuilder({}).color;
 
-        final textWithSelectionAttributions =
-            textSelection != null && _selectedTextColorStrategy != null && componentTextColor != null
-                ? (viewModel.text.copyText(0)
-                  ..addAttribution(
-                    ColorAttribution(_selectedTextColorStrategy!(
-                      originalTextColor: componentTextColor,
-                      selectionHighlightColor: _selectionStyles.selectionColor,
-                    )),
-                    SpanRange(textSelection.start, textSelection.end - 1),
-                    // The selected range might already have a color attribution. We want to override it
-                    // with the selected text color.
-                    overwriteConflictingSpans: true,
-                  ))
-                : viewModel.text;
+        final textWithSelectionAttributions = textSelection != null &&
+                !textSelection.isCollapsed &&
+                _selectedTextColorStrategy != null &&
+                componentTextColor != null
+            ? (viewModel.text.copyText(0)
+              ..addAttribution(
+                ColorAttribution(_selectedTextColorStrategy!(
+                  originalTextColor: componentTextColor,
+                  selectionHighlightColor: _selectionStyles.selectionColor,
+                )),
+                SpanRange(textSelection.start, textSelection.end - 1),
+                // The selected range might already have a color attribution. We want to override it
+                // with the selected text color.
+                overwriteConflictingSpans: true,
+              ))
+            : viewModel.text;
 
         viewModel
           ..text = textWithSelectionAttributions

super_editor/test/super_editor/supereditor_selection_test.dart

@@ -84,6 +84,27 @@ void main() {
         expect(richText.getSpanForPosition(const TextPosition(offset: 5))!.style!.color, Colors.green);
         expect(richText.getSpanForPosition(const TextPosition(offset: 16))!.style!.color, Colors.green);
       });
+
+      testWidgetsOnAllPlatforms("does not crash when triple tapping on an empty paragraph", (tester) async {
+        // This test is to ensure that the selection color strategy doesn't crash when the paragraph is empty.
+        // See https://github.com/superlistapp/super_editor/issues/2253 for more context.
+        final stylesheet = defaultStylesheet.copyWith(
+          selectedTextColorStrategy: ({required Color originalTextColor, required Color selectionHighlightColor}) {
+            return Colors.white;
+          },
+        );
+
+        await tester //
+            .createDocument()
+            .withSingleEmptyParagraph()
+            .useStylesheet(stylesheet)
+            .pump();
+
+        // Tripple tap on the empty paragraph.
+        await tester.tripleTapInParagraph("1", 0);
+
+        // Reaching this point means the editor didn't crash.
+      });
     });
 
     testWidgetsOnArbitraryDesktop("calculates upstream document selection within a single node", (tester) async {