in cursor.updateRow(row) RuntimeError: Object invalid or no longer set

I’m running an arcpy code in IDLE that update a field in shapefile. After updating it get the following error:

Traceback (most recent call last): File “C:/Users/10/Desktop/test.py”, line 21, in cursor.updateRow(row_st) RuntimeError: Object invalid or no longer set.

And, here is the code:

from arcpy.sa import *
import arcpy
from arcpy import env
env.workspace = r"D:Phd-Filesdataset.mdb"
env.overwriteOutput = True

fields_Street=["OBJECTID","SHAPE@","t_light_de", "st_dense", "bikeLn_den", "Buff_area", "len"]

cursor= arcpy.UpdateCursor("Boston_Street_Buff_10_Joint", fields_Street)
for row_st in cursor:
    geom = row_st.getValue('Shape')
    # calculate street density 
    _area_buff=0
    arcpy.Buffer_analysis(geom, "geom_buff", "10 Meters")
    cursor_buff= arcpy.SearchCursor("geom_buff", ["ORIG_FID", "Shape_Area"])
    for row_buff in cursor_buff:
        _area_buff=_area_buff+ row_buff.getValue('Shape_Area')
        row_st.setValue ('st_dense' , row_st.getValue('Buff_area')/_area_buff)
    del cursor_buff, row_buff
    print ('street_ObjectID={0}, bike_lane density= {1}'.format(row_st.getValue('OBJECTID'), row_st.getValue('st_dense')))
    cursor.updateRow(row_st)
    arcpy.Delete_management("geom_buff")