Fix 'Apply Selected Modifier' operator issues due to improper sequencing, document apply\_modifier a little bit and make it a little less error-prone

__init__.py

@@ -81,11 +81,21 @@ def copy_attributes(a, b):
 ######################################################
 
 def apply_modifier(target_object=None, target_modifiers=None):
+    """
+    Applies the given list of modifiers (or all if target_modifiers is None) to the given object.
+    If no object is supplied, the currently active object is assumed.
+    This function expects Object Mode, and changes the selection and active object.
+    """
     if target_object is None:
         obj_src = bpy.context.window.view_layer.objects.active
     else:
         obj_src = target_object
 
+    # Without this guard, direct invocation of apply_modifier can cause an odd situation.
+    # Even if the only selected object is the target, it causes a change in behaviour.
+    # An extra unwanted shape key, named after the object, is created.
+    bpy.ops.object.select_all(action='DESELECT')
+
     if target_modifiers is None:
         target_modifiers = []
         for x in obj_src.modifiers:
@@ -248,25 +258,26 @@ def poll(cls, context):
         return obj and obj.type == 'MESH'
 
     def execute(self, context):
-        obj = context.window.view_layer.objects.active
-        
+        obj = context.object
+
         if self.modifier_names and len(self.modifier_names) > 0:
             bpy.ops.object.select_all(action='DESELECT')
             str_targets = []
-            for i in range(len(self.modifier_names)):
+            usable_slots = min(len(self.modifier_names), 32)
+            for i in range(usable_slots):
                 if self.flags[i] and obj.modifiers[self.modifier_names[i]]:
                     str_targets.append(self.modifier_names[i])
 
             apply_modifier(target_object=obj, target_modifiers=str_targets)
 
             obj.select_set(True)
-        else:
-            self.modifier_names = tuple(i.name for i in obj.modifiers)
-            self.flags = tuple(False for i in range(32))
         return {'FINISHED'}
 
     def invoke(self, context, event):
         wm = context.window_manager
+        obj = context.object
+        self.modifier_names = tuple(i.name for i in obj.modifiers)
+        self.flags = tuple(False for i in range(32))
         return wm.invoke_props_dialog(self)
 
     def draw(self, context):
@@ -275,7 +286,8 @@ def draw(self, context):
         layout = self.layout
         col = layout.column()
 
-        for i in range(len(self.modifier_names)):
+        usable_slots = min(len(self.modifier_names), 32)
+        for i in range(usable_slots):
             col.prop(self, "flags", text=self.modifier_names[i], index=i)