default material switcher

import bpy
from bpy.props import BoolProperty

class SeqModalTimerOperator(bpy.types.Operator):
    """Operator which runs its self from a timer"""
    bl_idname = "wm.modal_timer_operator"
    bl_label = "Default Material Timer Operator"

    _timer = None
    mat_count = len(bpy.data.materials)

    def modal(self, context, event):
        scene = context.scene

        #if event.type in {'ESC'} or not scene.default_material_switch:
        if not scene.default_material_switch:
            self.cancel(context)
            return {'CANCELLED'}

        if event.type == 'TIMER':
            if len(bpy.data.materials) > self.mat_count:
                self.mat_count = len(bpy.data.materials)
                self.switch_nodes()

        return {'PASS_THROUGH'}

    def switch_nodes(self):
        #get active object's active material
        node_tree = bpy.context.active_object.active_material.node_tree
        #save the location of the diffuse node
        location = node_tree.nodes['Diffuse BSDF'].location.copy()
        #delete that node
        node_tree.nodes.remove(node_tree.nodes['Diffuse BSDF'])
        #add new Principled node
        principled = node_tree.nodes.new('ShaderNodeBsdfPrincipled')
        # move it to where the diffuse node used to be
        principled.location = location
        #make the connection
        node_tree.links.new(principled.outputs['BSDF'], node_tree.nodes['Material Output'].inputs['Surface'])

    def execute(self, context):
        wm = context.window_manager
        self._timer = wm.event_timer_add(0.1, context.window)
        wm.modal_handler_add(self)
        return {'RUNNING_MODAL'}

    def cancel(self, context):
        scene = context.scene
        wm = context.window_manager
        wm.event_timer_remove(self._timer)
        scene.default_material_switch = False
        print("modal ended", scene.default_material_switch)

def trigger_timmer(self, context):
    if self.default_material_switch:
        bpy.ops.wm.modal_timer_operator()


class DMPanel(bpy.types.Panel):
    """Creates a Panel in the Properties window"""
    bl_label = "Default Material Switcher"
    bl_idname = "DM_control"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "material"

    def draw(self, context):
        scene = context.scene
        if scene.default_material_switch:
            text = "Stop   Default Material"
            icon = "PAUSE"
        else:
            text = "Start   Default Material"
            icon = "PLAY"
        scene = context.scene
        layout = self.layout

        row = layout.row()
        row.prop(scene, "default_material_switch", text=text, icon=icon, toggle=True)


def register():
    bpy.utils.register_class(DMPanel)
    bpy.utils.register_class(SeqModalTimerOperator)
    bpy.types.Scene.default_material_switch = BoolProperty(default=False, name="Default Material Boolean", description="Turn on/off default material switcher", update=trigger_timmer)


def unregister():
    bpy.utils.unregister_class(SeqModalTimerOperator)
    bpy.utils.unregister_class(DMPanel)


if __name__ == "__main__":
register()