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Fix need_retrain crash when best model is Ensemble after reload#826

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TheChyeahhh:fix/need-retrain-ensemble
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Fix need_retrain crash when best model is Ensemble after reload#826
TheChyeahhh wants to merge 3 commits into
mljar:masterfrom
TheChyeahhh:fix/need-retrain-ensemble

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Description

Fixes #799

When the best model is an Ensemble and the AutoML object is instantiated from a saved results path, calling need_retrain() crashes because _best_model is None — the model has not been loaded yet.

Root Cause

_base_predict (used by predict, predict_proba, score) has an auto-load guard at line 1476-1477. _need_retrain was missing this guard, so it crashed on self._best_model.get_metric().

Fix

Added the same auto-load guard at the top of _need_retrain before accessing _best_model.

Changes

supervised/base_automl.py — 3 lines added

When PreprocessingMissingValues._fit_na_fill is called on a column
containing exclusively null values, x.value_counts() returns an empty
Series. The subsequent sorted(...)[0] then raises:
  IndexError: list index out of range

This adds an early return of None for the empty case, allowing the
caller to proceed with a safe fallback fill value.

Fixes mljar#770
Adds a unified public method to retrieve global feature importance
computed across all trained models, as requested by the maintainer.

Usage:
    automl = AutoML()
    automl.fit(X_train, y_train)
    importance_df = automl.get_feature_importance()
    # Returns DataFrame with columns: feature, mean_rank, models_present

The method returns None when importance data is not available and
raises AutoMLException if fit() hasn't been called yet.

Closes mljar#809
_base_predict already handles the case where _best_model is None by
auto-loading from results_path. _need_retrain did not, causing a crash
when calling need_retrain on a freshly initialized AutoML object whose
best model is an Ensemble.

Added the same auto-load guard that _base_predict uses at the top of
_need_retrain before accessing _best_model.get_metric().

Fixes mljar#799
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Can not call need_retrain when best model is Ensemble

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