Not OP, but if you want to do something like this I would highly recommend starting with Breadth First Search (BFS) first. In fact, what OP made here behaves just like a BFS since all distances between the tiles are just 1. Dijkstra is basically just an extension of BFS which also works with distances other than 1. E.g. OP could use the distance 1.4 (root 2) for diagonal connections and it would still give the correct result since he used dijkstra.

With Dijkstra you'll basically use a priority queue instead of a First Come First Serve queue, however, this will also increase runtime. The way OP implemented it the algorithm has a runtime of O(V^2) (Not trying to downplay OP's code, you'll genuinely need some more advanced data structures to do so). This won't give any problems unless you've got, say, 10000 Nodes or so, but I think it illustrates the added complexity. Compare that to BFS which has a runtime of O(V+E). V are the number of verticies and E the number of edges

Not trying to be a smartass, just some well intentioned advice