Adding Tempo text or clef within plugin crashes Musescore (2). What am I doing wrong?

I'm trying to write a plugin that can generate a melody - something like the random2 plugin, but with some constraints on what melody is generated.

User input is (among other things) the instrument.

What I would like to do is to set a clef appropriate for the selected instrument programmatically. But when I try to do that, Musescore crashes.

As basis for my code, I looked at the random2 plugin. This fragment creates a timesignature:

score.startCmd();
...
var cursor = score.newCursor();
cursor.track = 0;

cursor.rewind(0);

var ts = newElement(Element.TIMESIG);
ts.setSig(numerator, denominator);
cursor.add(ts);

My -naive- assumption is that something similar should work for adding a clef, so I wrote:

var cleff = newElement(Element.CLEF);
cursor.add(cleff);

But when I do that, Musescore crashes. Another problem is that the API documentation (inside the plugin creator) lists only 2 properties and no methods for a Clef Element - I would need to set the clef type somewhere, no?

I have a similar issue for the tempo. I want to set the tempo of the entire piece to 4 quarters per second. So I wrote:

var tempoText = newElement(Element.TEMPO_TEXT);
tempoText.tempo = 4.0;
cursor.add(tempoText);

again, musescore crashes.

What am I doing wrong?