Hello,

I'm trying to change a material of one `MeshInstance`

. Here is the code where the node is created, basically a `MeshInstance`

with two surfaces, where at the end of the code I created a function to change the material of one of them (there is the error):

```
class_name Tile
extends Spatial
var N : int
var iL : float
var oL : float
func _init(N=6, iL=0.9, oL=1):
self.N = N
self.iL = iL
self.oL = oL
func _ready():
_generate_tile(N, iL, oL)
func _generate_tile(N, iL, oL):
# generates the MeshInstance for the Tile
# N : int = number of corners
# iL : float = inner radius
# oL : float = outer radius
# MeshInstance for the Mesh
var meshInstance : MeshInstance
# Mesh for the tile
var mesh : Mesh
meshInstance = MeshInstance.new()
meshInstance.name = 'TMI'
# angle between the position vectors of two consecutive vertices of the hexagon
var angle : float = 2*PI/N
# pivot points
var polyVertices : PoolVector3Array = PoolVector3Array([
Vector3.BACK*iL,
Vector3.BACK*oL
])
# generate verteces, first inner ones, then outer ones
for i in range(1, N):
polyVertices.append(polyVertices[0].rotated(Vector3.UP, -i*angle))
polyVertices.append(polyVertices[1].rotated(Vector3.UP, -i*angle))
# SurfaceTool for the inner polygon
var iSurfaceTool : SurfaceTool = SurfaceTool.new()
# SurfaceTool for the borders of the polygon
var oSurfaceTool : SurfaceTool = SurfaceTool.new()
iSurfaceTool.begin(Mesh.PRIMITIVE_TRIANGLES)
oSurfaceTool.begin(Mesh.PRIMITIVE_TRIANGLES)
for i in range(2*N-2):
if i%2 == 0:
iSurfaceTool.add_vertex(Vector3.ZERO)
iSurfaceTool.add_vertex(polyVertices[i])
iSurfaceTool.add_vertex(polyVertices[i+2])
oSurfaceTool.add_vertex(polyVertices[i])
oSurfaceTool.add_vertex(polyVertices[i+1])
oSurfaceTool.add_vertex(polyVertices[i+3])
oSurfaceTool.add_vertex(polyVertices[i])
oSurfaceTool.add_vertex(polyVertices[i+3])
oSurfaceTool.add_vertex(polyVertices[i+2])
iSurfaceTool.add_vertex(Vector3.ZERO)
iSurfaceTool.add_vertex(polyVertices[2*N-2])
iSurfaceTool.add_vertex(polyVertices[0])
iSurfaceTool.generate_normals()
oSurfaceTool.add_vertex(polyVertices[2*N-2])
oSurfaceTool.add_vertex(polyVertices[2*N-1])
oSurfaceTool.add_vertex(polyVertices[1])
oSurfaceTool.add_vertex(polyVertices[2*N-2])
oSurfaceTool.add_vertex(polyVertices[1])
oSurfaceTool.add_vertex(polyVertices[0])
oSurfaceTool.generate_normals()
mesh = iSurfaceTool.commit()
mesh = oSurfaceTool.commit(mesh)
meshInstance.mesh = mesh
meshInstance.set_surface_material(1, MATERIALS.dict[1])
add_child(meshInstance, true)
func _change_center(key):
get_node('TMI').set_surface_material(0, MATERIALS.dict[key])
```

I instance this node in the scene via:

```
extends Spatial
func _ready():
var tile = Tile.new()
tile._change_center(2) # HERE IS THE PROBLEM
add_child(tile, true)
```

`MATERIALS`

contains a dictionary with the materials. I get the error when I try to change the material of one of the surfaces. I think that the is because `get_node('TMI')`

doesn't give the `MeshInstance`

child.