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ApiMain.php, line 373:

} catch ( Exception $e ) {

line 380:

if ( $wgLogExceptionBacktrace ) {
	wfDebugLog( 'exception', $e->getLogMessage() . "\n" . $e->getTraceAsString() . "\n" );
} else {
	wfDebugLog( 'exception', $e->getLogMessage() );
}

Since getLogMessage() is defined MWException,  if Exception caught is not MWException (which is allowed by catch), we get a call to undefined method fatal.