fix: guard against GridStack.init() returning null in gridstack 13

gridstack 13's types widen GridStack.init()'s return type to include
null; add the corresponding early return before using the instance.

Author: N-thnI

src/components/dashboard/DashboardGrid.tsx

@@ -148,6 +148,8 @@ export default function DashboardGrid({
         },
       });
 
+      if (!grid) return;
+
       gridRef.current = grid;
 
       grid.on("change", () => {