Fix bug that causes plot functions to visibly return NULL, closes #51

DESCRIPTION

 Package: stressaddition
 Type: Package
 Title: Modelling Tri-Phasic Concentration-Response Relationships
-Version: 3.0.1
-Date: 2020-07-29
+Version: 3.0.2
+Date: 2020-08-18
 Authors@R: c(person("Sebastian", 
                     "Henz", 
                     role = c("aut", "cre"), 

R/plot_stress.R

@@ -194,5 +194,5 @@ plot_stress <- function(model,
             )
         }
     }
-    NULL  # suppress all possible return values
+    invisible(NULL)  # suppress all possible return values
 }

R/plot_survival.R

@@ -175,5 +175,5 @@ plot_survival <- function(model,
             )
         }
     }
-    NULL  # suppress all possible return values
+    invisible(NULL)  # suppress all possible return values
 }