I have a camera and added a MeshInstance node to the Spatial node and attached this script. Why am I not seeing any triangle yet?

extends MeshInstance
 
func _ready():
    var size = 10
    var st = SurfaceTool.new()
    var mat = SpatialMaterial.new()
    mat.albedo_color = Color(1.0, 0.0, 0.0)
    st.set_material(mat)
    st.begin(Mesh.PRIMITIVE_TRIANGLES)
    st.add_uv(Vector2(0, 0))
    st.add_vertex(Vector3(-size, -size,  0))
    st.add_uv(Vector2(1, 1))
    st.add_vertex(Vector3( size,  size,  0))
    st.add_uv(Vector2(1, 0))
    st.add_vertex(Vector3( size, -size,  0))
    st.generate_normals()
    var mesh = st.commit()
    self.set_mesh(mesh)

Using Godot3, I have a camera and added a MeshInstance node to the Spatial node and attached this script. Why am I not seeing any triangle yet?

extends MeshInstance
 
func _ready():
    var size = 10
    var st = SurfaceTool.new()
    var mat = SpatialMaterial.new()
    mat.albedo_color = Color(1.0, 0.0, 0.0)
    st.set_material(mat)
    st.begin(Mesh.PRIMITIVE_TRIANGLES)
    st.add_uv(Vector2(0, 0))
    st.add_vertex(Vector3(-size, -size,  0))
    st.add_uv(Vector2(1, 1))
    st.add_vertex(Vector3( size,  size,  0))
    st.add_uv(Vector2(1, 0))
    st.add_vertex(Vector3( size, -size,  0))
    st.generate_normals()
    var mesh = st.commit()
    self.set_mesh(mesh)