Pred$Learner is not updated after changing the learner in the Learner tab

[karo702]

If a new learner is selected in the Learner tab after another learner has previously been trained in the Predict tab, Pred$Learner remains unchanged and continues to point to the old learner.

Steps for reproducibility:

1. Select decision tree as Learner 1 in the Learner tab.
2. Train the learner in the Predict tab.
3. Go back to the Learner tab and change Learner 1 to random forest
4. Train the learner again in the Predict tab

-> Pred$Learner is still rpart (see, for example, code extraction)

Cause: server_predict.R line 313