disable placing text if not possible

__init__.py

@@ -277,6 +277,16 @@ class FONT3D_PT_TextPlacement(bpy.types.Panel):
     bl_space_type = "VIEW_3D"
     bl_region_type = "UI"
 
+    can_place = False
+
+    @classmethod
+    def poll(self, context):
+        if type(context.active_object) != type(None) and context.active_object.type == 'CURVE':
+            self.can_place = True
+        else:
+            self.can_place = False
+        return True
+
     def draw(self, context):
         layout = self.layout
         wm = context.window_manager
@@ -289,7 +299,12 @@ class FONT3D_PT_TextPlacement(bpy.types.Panel):
         layout.row().prop(font3d, "text")
         layout.row().prop(font3d, "letter_spacing")
         layout.column().prop(font3d, "orientation")
-        layout.row().operator('font3d.placetext', text='Place Text')
+        placerow = layout.row()
+        placerow.enabled = self.can_place
+        placerow.operator('font3d.placetext', text='Place Text')
+        if not self.can_place:
+            layout.label(text="Cannot place Text.")
+            layout.label(text="Select a curve as active object.")
 
 class FONT3D_PT_TextManagement(bpy.types.Panel):
     bl_label = "Text Management"
@@ -474,7 +489,7 @@ class FONT3D_OT_TemporaryHelper(bpy.types.Operator):
         return {'FINISHED'}
 
 class FONT3D_OT_PlaceText(bpy.types.Operator):
-    """Place Text 3D"""
+    """Place Text 3D on active object"""
     bl_idname = f"{__name__}.placetext"
     bl_label = "Place Text"
     bl_options = {'REGISTER', 'UNDO'}