hi, i received this error while trying to assign a vertex property map to a graph, and i'm not sure how to get around it. context: i am running minimize_blockmodel_dl() many times in parallel using microprocessing. because my graph is large, i don't want it to save multiple copies of the graph. so i am trying to only keep the block id each time. however, when i try to do this, i get the above error here's a somewhat simplified example: ``` import graph_tool.all as gt import multiprocessing as mp pool = mp.Pool(5) def fit_sbm(): state = gt.minimize_blockmodel_dl(g) b = state.get_blocks() return b blocks = pool.map(fit_sbm, range(5)) pool.close b0 = blocks[0] membership = g.new_vertex_property("int") membership = b0 g.vertex_properties["membership"] = b0 ``` could you explain why this error comes up, and also if possible suggest an alternative way to proceed? thanks, -sam