Error in sum: invalid 'type' (list) of argument

I'm afraid that in order to help you , I would need a reprex. As I'm unable to recreate your issue without it.

I took the code and initial data that you originally provided, and satisfied myself that there is nothing inherintly wrong with your apprpoach, it works after all. I suppose the devil is in the details which I can't access as of yet.

Here is that test shiny app I created.
The top plot is unfiltered, the bottom one is filtered on State code (iso..)


library(shiny)
library(leaflet)
library(sp)
library(albersusa) #devtools::install_github("hrbrmstr/albersusa")
library(rmapshaper)

spdf <- rmapshaper::ms_simplify(usa_sf(), keep = 0.1)
pal <- colorNumeric("Blues", domain = spdf$pop_2014)
epsg2163 <- leafletCRS(
  crsClass = "L.Proj.CRS",
  code = "EPSG:2163",
  proj4def = "+proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +a=6370997 +b=6370997 +units=m +no_defs",
  resolutions = 2^(16:7))

# Define UI for application that draws a histogram
ui <- fluidPage(
  
  # Application title
 
  titlePanel("leaflettest"),
  sidebarLayout(sidebarPanel (
    selectInput(inputId = "iso_3166_2",label="Filter on iso_3166_2",
                choices = unique(pull(spdf,iso_3166_2)))
  ),
              mainPanel (  
  leafletOutput("leafplot1"),
  leafletOutput("leafplot2")
              )
))

# Define server logic required to draw a histogram
server <- function(input, output) {
  
  unfilteredData1 <- reactive({
   spdf
  })
  filteredData2 <- reactive({
    spdf[spdf$iso_3166_2 == input$iso_3166_2,]
  })

  output$leafplot1 <- renderLeaflet({
    
    leaflet(unfilteredData1(), options = leafletOptions(crs = epsg2163)) %>%
      addPolygons(weight = 1, color = "#444444", opacity = 1,
                  fillColor = ~pal(pop_2014), fillOpacity = 0.7, smoothFactor = 0.5,
                  label = ~paste(name, pop_2014),
                  labelOptions = labelOptions(direction = "auto"))
  })
  output$leafplot2 <- renderLeaflet({
    
    leaflet(filteredData2(), options = leafletOptions(crs = epsg2163)) %>%
      addPolygons(weight = 1, color = "#444444", opacity = 1,
                  fillColor = ~pal(pop_2014), fillOpacity = 0.7, smoothFactor = 0.5,
                  label = ~paste(name, pop_2014),
                  labelOptions = labelOptions(direction = "auto"))
  })
}

# Run the application 
shinyApp(ui = ui, server = server)