Change type of SizeAttribute from String to double

app/assets/sample_data.json

@@ -473,6 +473,30 @@
       },
       "insert": "Huge"
    },
+   {
+      "attributes": {
+         "size": "15.0"
+      },
+      "insert": "font size 15"
+   },
+   {
+      "insert": " "
+   },
+   {
+      "attributes": {
+         "size": "35"
+      },
+      "insert": "font size 35"
+   },
+   {
+      "insert": " "
+   },
+   {
+      "attributes": {
+         "size": "20"
+      },
+      "insert": "font size 20"
+   },
    {
       "insert": "\n"
    }

lib/widgets/text_line.dart

@@ -153,6 +153,10 @@ class TextLine extends StatelessWidget {
           res = res.merge(defaultStyles.sizeHuge);
           break;
         default:
+          double fontSize = double.tryParse(size.value);
+          if (fontSize != null) {
+            res = res.merge(TextStyle(fontSize: fontSize));
+          } else
             throw "Invalid size ${size.value}";
       }
     }

lib/widgets/toolbar.dart

@@ -742,6 +742,9 @@ class _HistoryButtonState extends State<HistoryButton> {
   }
 
   void _setIconColor() {
+
+    if(!mounted) return;
+
     if (widget.undo) {
       setState(() {
         _iconColor = widget.controller.hasUndo