plot: fix R4 visualization

Author: M3nin0

pysits/visualization/base.py

@@ -104,7 +104,8 @@ def _base_plot(
         # Assuming a plot is a list of elements, we always have many elements.
         # Cases where we have a single element indicates that the plot object is
         # inside a list.
-        while len(plots) == 1:
+        # This is only true when the object is not a ggplot object
+        while "ggplot2::ggplot" not in list(plots.rclass) and len(plots) == 1:
             plots = plots[0]
 
         # Create a temporary file