More robustness and informative errors in grid

com.oracle.truffle.r.library/src/com/oracle/truffle/r/library/fastrGrid/GridContext.java

@@ -98,11 +98,12 @@ public final class GridContext {
     }
 
     public void setCurrentDevice(String name, GridDevice currentDevice, String filenamePattern) {
+        assert devices.size() == RGridGraphicsAdapter.getDevicesCount() : devices.size() + " vs " + RGridGraphicsAdapter.getDevicesCount();
         RGridGraphicsAdapter.addDevice(name);
         RGridGraphicsAdapter.setCurrentDevice(name);
         currentDeviceIdx = this.devices.size();
         this.devices.add(new DeviceAndState(currentDevice, filenamePattern));
-        assert devices.size() == RGridGraphicsAdapter.getDevicesCount();
+        assert devices.size() == RGridGraphicsAdapter.getDevicesCount() : devices.size() + " vs " + RGridGraphicsAdapter.getDevicesCount();
     }
 
     public void openDefaultDevice() {

com.oracle.truffle.r.library/src/com/oracle/truffle/r/library/fastrGrid/Unit.java

@@ -221,7 +221,7 @@ public final class Unit {
             case STRINGWIDTH:
             case MYSTRINGWIDTH:
                 str = RRuntime.asString(data.getDataAt(0));
-                return value * GridUtils.getStringWidth(ctx.gpar.getDrawingContext(index), ctx.device, str);
+                return value * GridUtils.getStringWidth(ctx.gpar.getDrawingContext(index), ctx.device, str == null ? "" : str);
             case STRINGHEIGHT:
             case MYSTRINGHEIGHT:
                 str = RRuntime.asString(data.getDataAt(0));