Try this:

diff --git a/libs/flake/text/KoFontRegistry.cpp b/libs/flake/text/KoFontRegistry.cpp
index 72bc9f7f10..fae75e87bf 100644
--- a/libs/flake/text/KoFontRegistry.cpp
+++ b/libs/flake/text/KoFontRegistry.cpp
@@ -741,6 +741,7 @@ KoSvgText::FontMetrics KoFontRegistry::fontMetricsForCSSValues(KoCSSFontInfo inf
             yRes,
             disableFontMatching,
             language);
+        if (faces.empty()) return metrics;
         metrics = KoFontRegistry::generateFontMetrics(faces.front(), isHorizontal);
         d->fontMetrics().insert(suggestedHash, metrics);
     }

May also need:

diff --git a/libs/flake/text/KoSvgTextProperties.cpp b/libs/flake/text/KoSvgTextProperties.cpp
index 639a0bbd79..75b26fc326 100644
--- a/libs/flake/text/KoSvgTextProperties.cpp
+++ b/libs/flake/text/KoSvgTextProperties.cpp
@@ -1034,7 +1034,10 @@ qreal KoSvgTextProperties::xHeight() const
     const qreal fontSizeVal = fontSize().value;
     const bool isHorizontal = propertyOrDefault(WritingModeId).toInt() == KoSvgText::HorizontalTB;
     const KoSvgText::FontMetrics metrics = KoFontRegistry::instance()->fontMetricsForCSSValues(info, isHorizontal);
-    return double((metrics.xHeight*-1)/metrics.fontSize) * fontSizeVal;
+    if (metrics.xHeight > 0 && metrics.fontSize > 0) {
+        return double((metrics.xHeight*-1)/metrics.fontSize) * fontSizeVal;
+    }
+    return fontSizeVal*0.5;
 }

To avoid division by 0.