check divide by 0

Signed-off-by: fufesou <shuanglongchen@yeah.net>

flutter/lib/models/model.dart

@@ -810,6 +810,10 @@ class CanvasModel with ChangeNotifier {
   double get tabBarHeight => stateGlobal.tabBarHeight;
 
   moveDesktopMouse(double x, double y) {
+    if (size.width == 0 || size.height == 0) {
+      return;
+    }
+
     // On mobile platforms, move the canvas with the cursor.
     final dw = getDisplayWidth() * _scale;
     final dh = getDisplayHeight() * _scale;