Blender crashing

# LUCID JOINT ADDON

import bpy
import bgl
from bpy.app.handlers import persistent
from bpy.types import Operator
from bpy.props import FloatVectorProperty, BoolProperty
from bpy_extras.object_utils import AddObjectHelper, object_data_add
from mathutils import Vector


def draw_callback_px(self, context):
    pass
    # # 50% alpha, 2 pixel width line
    # bgl.glEnable(bgl.GL_BLEND)
    # bgl.glColor4f(0.0, 0.0, 0.0, 0.5)
    # bgl.glLineWidth(2)

    # bgl.glBegin(bgl.GL_LINE_STRIP)
    # for obj in bpy.data.objects:
    #     l = obj.location
    #     try:
    #         c  = obj.constraints
    #         if "Child Of" in c:
    #             l = c["Child Of"].target.location + l
    #     except AttributeError as ex:
    #         pass
    #     bgl.glVertex3f(l.x, l.y, l.z)
    # bgl.glEnd()

    # # restore opengl defaults
    # bgl.glLineWidth(1)
    # bgl.glDisable(bgl.GL_BLEND)
    # bgl.glColor4f(0.0, 0.0, 0.0, 1.0)

def add_object(self, context):
    def unselect_all():
        bpy.ops.object.select_all(action="DESELECT")

    selected = bpy.context.selected_objects
    if len(selected) is not 2:
        return True

    'FINISHED' in bpy.ops.object.empty_add(type='PLAIN_AXES', location=(0, 0, 0))
    joint = bpy.context.active_object
    'FINISHED' in bpy.ops.object.empty_add(type='PLAIN_AXES', location=(0, 0, 0))
    a = bpy.context.active_object
    'FINISHED' in bpy.ops.object.empty_add(type='PLAIN_AXES', location=(0, 0, 0))
    b = bpy.context.active_object

    a.protected = True
    b.protected = True

    joint.select = True
    a.select = True
    b.select = True
    bpy.context.scene.objects.active = joint
    bpy.ops.object.parent_set()

    unselect_all()

    bpy.context.scene.objects.active = a
    selected[0].select = True
    bpy.ops.object.constraint_add_with_targets(type="CHILD_OF")

    unselect_all()

    bpy.context.scene.objects.active = b
    selected[1].select = True
    bpy.ops.object.constraint_add_with_targets(type="CHILD_OF")


    # return {'RUNNING_MODAL'}

    return False


class OBJECT_OT_add_object(Operator, AddObjectHelper):
    """Create a new LUCID Joint"""
    bl_idname = "mesh.add_b2joint"
    bl_label = "Add LUCID Joint"
    bl_options = {'REGISTER', 'UNDO'}

    @classmethod
    def register(cls):
        # the arguments we pass the the callback
        # args = (self, context)
        # Add the region OpenGL drawing callback
        # draw in view space with 'POST_VIEW' and 'PRE_VIEW'
        pass



    def modal(self, context, event):
        context.area.tag_redraw()

        if event.type in {'ESC'}:
            bpy.types.SpaceView3D.draw_handler_remove(self._handle, 'WINDOW')
            return {'CANCELLED'}

        return {'PASS_THROUGH'}

    def execute(self, context):
        print("QWEQWEQWEQWEQWEQWEQWE")
        self._handle = bpy.types.SpaceView3D.draw_handler_add(draw_callback_px, (), 'WINDOW', 'POST_VIEW')
        bpy.context.window_manager.modal_handler_add(self)
        return {'FINISHED'}

    def invoke(self, context, event):
        return {'CANCELLED'} if add_object(self, context) else {'FINISHED'}


# Registration

def add_object_button(self, context):
    self.layout.operator(
        OBJECT_OT_add_object.bl_idname,
        text="LUCID Joint",
        icon='PLUGIN')

#Protectedness follows
bpy.types.Object.protected = BoolProperty(name = 'protected', default = False)
def main_prot(context):
    for obj in context.selected_objects:
        if not obj.protected :
            bpy.context.scene.objects.unlink(obj)
            bpy.data.objects.remove(obj)
        else :
            print(obj.name +' is protected')


class delete_override(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.delete"
    bl_label = "Simple Object Operator"

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def execute(self, context):
        main_prot(context)
        return {'FINISHED'}

@persistent
def load_handler(dummy):
    bpy.ops.mesh.add_b2joint()

def register():
    bpy.utils.register_class(delete_override)
    bpy.utils.register_class(OBJECT_OT_add_object)
    bpy.types.INFO_MT_mesh_add.append(add_object_button)
    bpy.app.handlers.load_post.append(load_handler)
    bpy.ops.mesh.add_b2joint()

def unregister():
    bpy.utils.unregister_class(delete_override)
    bpy.utils.unregister_class(OBJECT_OT_add_object)
    bpy.types.INFO_MT_mesh_add.remove(add_object_button)

if __name__ == "__main__":
    register()