nil pointer dereference after removing all nodes

[Schuwi]

When removing all nodes from a Graph and then adding a node again a nil pointer dereference error occurs:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x4964e9]

goroutine 1 [running]:
github.com/coder/hnsw.(*Graph[...]).Dims(...)
        /home/schuwi/go/pkg/mod/github.com/coder/[email protected]/graph.go:320
github.com/coder/hnsw.(*Graph[...]).assertDims(0x46892f?, {0x565404?, 0x1?, 0xc0001281b0})
        /home/schuwi/go/pkg/mod/github.com/coder/[email protected]/graph.go:308 +0xa9
github.com/coder/hnsw.(*Graph[...]).Add(0x4e5ba0, {0xc000120f20, 0xc000062700?, 0x466b39?})
        /home/schuwi/go/pkg/mod/github.com/coder/[email protected]/graph.go:334 +0x9c
main.main()
        /tmp/go_hnsw_bug/main.go:18 +0x139
exit status 2

Minimum reproducible example:

package main

import (
	"github.com/coder/hnsw"
)

var vec = []float32{1}

func main() {
	g := hnsw.NewGraph[int]()
	g.Add(hnsw.MakeNode(1, vec))
	g.Delete(1)

	// This call will fail on a nil pointer dereference
	g.Add(hnsw.MakeNode(1, vec))
}