Fix use of CoerceCaretIndex on Text change

src/Avalonia.Controls/TextBox.cs

@@ -214,9 +214,9 @@ namespace Avalonia.Controls
                 if (!_ignoreTextChanges)
                 {
                     var caretIndex = CaretIndex;
-                    SelectionStart = CoerceCaretIndex(SelectionStart, value?.Length ?? 0);
+                    SelectionStart = CoerceCaretIndex(SelectionStart, value);
-                    SelectionEnd = CoerceCaretIndex(SelectionEnd, value?.Length ?? 0);
+                    SelectionEnd = CoerceCaretIndex(SelectionEnd, value);
-                    CaretIndex = CoerceCaretIndex(caretIndex, value?.Length ?? 0);
+                    CaretIndex = CoerceCaretIndex(caretIndex, value);
 
                     if (SetAndRaise(TextProperty, ref _text, value) && !_isUndoingRedoing)
                     {
@@ -677,11 +677,15 @@ namespace Avalonia.Controls
             }
         }
 
-        private int CoerceCaretIndex(int value) => CoerceCaretIndex(value, Text?.Length ?? 0);
+        private int CoerceCaretIndex(int value) => CoerceCaretIndex(value, Text);
 
-        private int CoerceCaretIndex(int value, int length)
+        private int CoerceCaretIndex(int value, string text)
         {
-            var text = Text;
+            if (text == null)
+            {
+                return 0;
+            }
+            var length = text.Length;
 
             if (value < 0)
             {
@@ -691,7 +695,7 @@ namespace Avalonia.Controls
             {
                 return length;
             }
-            else if (value > 0 && text[value - 1] == '\r' && text[value] == '\n')
+            else if (value > 0 && text[value - 1] == '\r' && value < length && text[value] == '\n')
             {
                 return value + 1;
             }