How to remove gridlines from interact_plot

I'm trying to remove gridlines from an interact_plot in R Studio, which I understand uses plotting from ggplot2.

I have tried the following code from ggplot2:

interact_plot(
  MC_Feat_Sit, 
  pred =  Mean_Mor_Con2, 
  modx = Agreement2, 
  modx.values = c(1, 4, 7), 
  x.label = "Moral Conviction", 
  y.label = "Situational Attributions", 
  legend.main = "Agreement", 
  modx.labels = c("Strongly Disagree", "Neutral", "Strongly Agree"), 
  theme(panel.grid = element_blank())
)

But I get this error message:

Mean_Mor_Con2 and Agreement2 and theme(panel.grid = element_blank()) are not included in an interaction with one another in the model.Error in names(modxvals2) <- modx.labels : attempt to set an attribute on NULL

I also get the same error message when trying to use a theme (e.g. theme_bw()).

Any help is much appreciated!

This is the graph I'm getting

The function interact_plot returns an object of type ggplot and then it can be modified afterwards and not directly inside this function:

plt <- interact_plot(
  MC_Feat_Sit, 
  pred =  Mean_Mor_Con2, 
  modx = Agreement2, 
  modx.values = c(1, 4, 7), 
  x.label = "Moral Conviction", 
  y.label = "Situational Attributions", 
  legend.main = "Agreement", 
  modx.labels = c("Strongly Disagree", "Neutral", "Strongly Agree")
)

plt <- plt + theme(panel.grid.major = element_blank())

plt